Here's one list I found. Of course, there is always Dunhill. It's been a while since I've been to London, one of my favorite cities in the world, but the last time I was there It was difficult to fine an old time Mom & Pop B&M. I did find one in Carnaby Street run by an older gent. Good luck and have fun.
B M Patel
Tobacconists - Retail (Shops & other Retail Outlets) based in the Clapham South Station area of London
Tel:
Address: Clapham South Station, Balham Hill, London, SW12 9DU
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Broadweighs
Tobacconists - Retail (Shops & other Retail Outlets) based in the Broad Street area of Teddington
Tel: 020 89773793
Address: 48 Broad Street, Teddington, Middlesex, TW11 8QY
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Canvasbay
Tobacconists - Retail (Shops & other Retail Outlets) based in the High Street area of Uxbridge
Tel: 01895 230709
Address: 171 High Street, Uxbridge, Middlesex, UB8 1JZ
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Davys
Tobacconists - Retail (Shops & other Retail Outlets) based in the The Market area of London
Tel: 020 78361415
Address: 27a The Market, The Piazza, London, , WC2E 8RD
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Fox
Tobacconists - Retail (Shops & other Retail Outlets) based in the St. James's Street area of London
Tel: 020 74939009
Address: 19 St. James's Street, London, , SW1A 1ES
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Freeway
Tobacconists - Retail (Shops & other Retail Outlets) based in the High Road area of Harrow
Tel: 020 84273074
Address: 361 High Road, Harrow, Middlesex, HA3 5EQ
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J J Fox (St James's)
Tobacconists - Retail (Shops & other Retail Outlets) based in the St James's Street area of London
Tel: 020 79303787
Address: 19 St James's Street, London, , SW1A 1ES
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Jeffersons Newsplus
Tobacconists - Retail (Shops & other Retail Outlets) based in the Wood Street area of London
Tel: 020 85211802
Address: 166 Wood Street, London, , E17 3HX
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K Markaou
Tobacconists - Retail (Shops & other Retail Outlets) based in the West Green Road area of London
Tel: 020 88882024
Address: 464 West Green Road, London, , N15 3PT
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Mithas
Tobacconists - Retail (Other) based in the Marylebone Road area of London
Tel: 020 74872728
Address: Baker Street Station, Marylebone Road, London, , NW1 5LA
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Monte's Cigar Store
Tobacconists - Retail (Shops & other Retail Outlets) based in the Halkin Arcade area of London
Tel: 020 72450890
Address: 16 Halkin Arcade, London, , SW1X 8JT
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Murrays
Tobacconists - Retail (Shops & other Retail Outlets) based in the Hoe Street area of London
Tel: 020 85206408
Address: 214 Hoe Street, London, , E17 3AY
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Regal Cigars
Tobacconists - Retail (Office & Administration) based in the Station Road area of Harrow
Tel: 020 88615238
Address: 60a Station Road, North Harrow, Harrow, Middlesex, HA2 7SL
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Rothmans Of Pall Mall
Tobacconists - Retail (Shops & other Retail Outlets) based in the Pall Mall area of London
Tel:
Address: 65 Pall Mall, London, SW1Y 5HZ
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Shears
Tobacconists - Retail (Shops & other Retail Outlets) based in the Chiswick High Road area of London
Tel: 020 89944820
Address: 298A Chiswick High Road, London, , W4 1PB
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Simplycigar
Tobacconists - Retail (Business at Home) based in the Tabor Road area of London
Tel: 020 88347123
Address: Tabor Road, London, , W6 0BN
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Sky Lark
Tobacconists - Retail (Shops & other Retail Outlets) based in the Vicarage Lane area of London
Tel: 020 85524324
Address: 11 Vicarage Lane, London, , E6 6AD
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The Candy Shop
Tobacconists - Retail (Shops & other Retail Outlets) based in the Station Road area of Harrow
Tel: 020 84277796
Address: 188 Station Road, Harrow, Middlesex, HA1 2RH
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The Kiosk
Tobacconists - Retail (Shops & other Retail Outlets) based in the High Street North area of London
Tel: 020 84724758
Address: 64 High Street North, London