For more information about the show (both past and present), visit these sites:
  Doctor Who: The Official Site
  Outpost Gallifrey

I have tried most of these addresses with success, however, I do NOT claim that they will work all the time.  Check out which ones worked for me here.

BAKER, COLIN (The Sixth Doctor)
c/o Evans and Reiss
100 Fawe Park Road
London SW15 2EA
-or-
Grafton House
2-3 Golden Square
#42-43
London W1R 3AD

BAKER, TOM (The Fourth Doctor)
c/o London Management
2-4 Noel Street
London, W1V 3RB

DAVISON, PETER (The Fifth Doctor)
18-21 Germyn Street
3rd Floor
London SW1Y 6HP

MCCOY, SYLVESTER (The Seventh Doctor)
111 Soutwick Mews
London W2 1JG
-or-
c/o Silvester Management
122 Wardour Street
London W1V 3LA

MCGANN, PAUL (The Eighth Doctor)
c/o Marina Martin Associates Ltd.
12-13 Poland Street
London W1V 3DE
-or-
6A Denbury Street
London, N1 8JU

ALDRED, SOPHIE ("Ace")
c/o John Grantham
5 Blenheim Street
London W1Y 9LB

BRIERLY, DAVID (second voice of "K9")
c/o Vincent Shaw Associates
20 Jay Mews
London SW7 2EP

BRYANT, NICOLA ("Peri Brown")
c/o Roger Carey Associates
31 Kensington Church Street
W8 4LL

FIELDING, JANET ("Tegan Jovanka")
c/o International Artists Drama
Mezzanine Floor
235 Regent Street
London W1R 8AX

FORD, CAROLE ANN ("Susan Foreman")
c/o Emko Productions
The Ridge
Hendon Wood Lane
NW7 4HR

HINES, FRASER ("Jamie McCrimmon")
c/o Peter Charlesworth
2nd Floor
68 Old Brompton Road
London SW7 3LQ

JAMESON, LOUISE ("Leela")
c/o Conway, van Gelder, Robinson
3rd Floor
18-21 Jermyn Street
London SW1Y 6HP

JOHN, CAROLINE ("Liz Shaw")
*** I do not have a valid address for her at this time ***

LANE, JACKIE ("Dodo Chaplet")
c/o Jackie Lane Ad Voice
Paramount House
162/170 Wardour Street
London WlV 3AT

LANGFORD, BONNIE ("Mel Bush")
c/o Michael Ladkin Personal Management
Suite 1
Ground Floor
1 Duchess Street
London W1N 3DE

LEESON, JOHN ("K9")
c/o Sheila Bourne Management
266 Greenwich Business Centre
Greenwich High Road
London SEI0 8JL

MANNING, KATY ("Josephine 'Jo' Grant")

MARTER, IAN ("Surgeon Lt. Harry Sullivan")

O'BRIEN, MAUREEN ("Vicki")
c/o Kate Feast Management
10 Primrose Hill Studios
Fitzroy Road
London NW1 8TR

PADBURY, WENDY ("Zoe Herriot")
c/o Evans and Reiss
100 Fawe Park Road
London SW15 2EA

PURVES, PETER ("Steven Taylor")
c/o Downes Presenters Agency
96 Broadway
Bexley Heath DA6 7DE

RUSSEL, WILLIAM ("Ian Chesterton")
c/o James Sharkey Associates Ltd.
21 Golden Square
London W1R 3PA

SLADEN, ELISABETH ("Sarah Jane Smith")
c/o Roger Carey Associates
31 Kensington Church Street
London W8 4LL
-or-
Burlington House
64 Chestwick High Road
London W4 1SY

STRICKSON, MARK ("Vislor Turlough")
c/o Evans and Reiss
100 Fawe Park Road
London SW15 2EA

SUTTON, SARAH ("Nyssa of Traken")
c/o Frazer-Skemp Management
34 Bramerton Street
London SW3 5LA

TAMM, MARY (first "Romana")
c/o Langford Associates
17 Westfields Avenue
London SW13 0AT

WARD, LALLA (second "Romana")
c/o London Management
2-4 Noel Street
London W1V 3RB

WATERHOUSE, MATTHEW ("Adric")
c/o Sandra Boyce
1 Kingsway House
Albion Road
London N16 0TA

WATLING, DEBORAH ("Victoria")
c/o Collis Management
182 Trevelyan Road
London SW17 9LW

WILLS, ANNEKE ("Polly Wright")

COURTNEY, NICHOLAS ("Brigadier Allistair Gordon Lethbridge-Stewart")
c/o Felix De Wolfe
1 Southampton Street
London WC2R 0LR

FRANKLIN, RICHARD ("Captain Mike Yates")
c/o Eric Glass
28 Berkeley Square
London WlX 6HD

LEVENE, JOHN ("Sergeant John Benton")

AINLEY, ANTHONY("The Master")
c/o Incoming Mail
BBC Television Centre
Wood Lane
London W12 7RJ

BANKS, DAVID ("Cyber Leader")
c/o Shane Collins Associates
2-5 Stedham Place
Bloomsbury
London WC1A 1BU

GOODERSON, DAVID (second "Davros")
c/o Marjorie Abel Ltd.
50 Maddox Street
London W1R 9PA

GOUGH, MICHAEL ("Celestial Toymaker")
c/o Fraser and Dunlop
Fifth Floor
The Chambers
Chelsea Harbour
Lots Road
London SW10 0XF

JAYSTON, MICHAEL ("The Valeyard" in Trial of a Timelord)
c/o Michael Whitehall Ltd.
125 Gloucester Road
London SW7

MOLLOY, TERRY ("Davros")
c/o Sue Hanmer Personal Management
Otterbourne House
Chobham Road
Ottershore
Chertsey
Surrey KT16 0QF

O'MARA, KATE ("The Rani" in Mark of the Rani and Time and the Rani)
c/o Michael Ladkin Personal Management
Suite 1
Ground Floor
1 Duchess Street
London W1N 3DE

SELBY, TONY ("Sabalom Glitz")
c/o AIM
5 Denmark Street
London WC2H 8LP

THORNE, STEPHEN (second "Omega")
c/o Thomas & Benda Associates Ltd.
361 Edgware Road
London W2 1BS

LAMBERT, VERITY (First Doctor Who producer)
c/o Cinema Verity
Cannon-Elstree Studios
Borehamwood
Herts WD6 1JG

BRUCE, ANGELA ("Brigadier Bambera")
c/o Susan Angel Associates Ltd.
1st Floor
D'Arblay Street
London SW1 3AX

CLUNES, MARTIN ("Lon" in Snakedance)
c/o ICM
Oxford House
76 Oxford House
London W1N 0AX

DARROW, PAUL ("Tekker" in Timelash)
c/o Silvester Management
122 Wardour Street
London W1V 3LA

DUNLOP, LESLEY ("Norna" in Frontios, "Susan Q" in The Happiness Patrol)
c/o Annette Stone Associates
9 Newburgh Street
London W1V 1LH

ELLIS, JANET ("Teka" in Horns of Nimon)
c/o Noel Gay Artists
22 Dean Street
London W1V 5AL

GREENE, SARAH ("Varne" in Attack of the Cybermen)
c/o Conway, van Gelder, Robinson
3rd Floor
19-21 Jermyn Street
London SW1Y 6HP

KEATING, MICHAEL ("Goudry" in The Sunmakers)
c/o Narrow Road Company
21/22 Poland Street
London W1V 3DD

KWOUK, BURT ("Lin Futu" in Four to Doomsday)
c/o London Management
2-4 Noel Street
London W1V 3RB

MARSH, JEAN ("Sara Kingdom" in The Daleks' Master Plan,"Morgaine" in Battlefield)
c/o London Management
2-4 Noel Street
London W1V 3RB
-or-
Guild House
Upper St.
Martin's Lane
London WC2H 9EG
-or-
c/o Artist Mail
BBC Information
P.O. Box 1116
Belfast BT2 7AJ
NORTHERN IRELAND

MAYNE, BELINDA ("Delta" in Delta and the Bannermen)
c/o CCA Management
4 Court Lodge
48 Sloane Square
London SW1W 8AT

SILVA, ADELE ("Squeak" in Survival)
c/o Emmerdale Production Centre
27 Burley Road
Leeds LS3 1JT

SIMS, JOAN ("Katryca" in Trial of a Timelord)
c/o MGA
Concorde House
18 Margaret Street
Brighton BN2 1TS