Thai grocery in London?
If you feel like travelling to North London, Oriental City (399 Edgware Rd, NW9) has a large selection of Thai groceries (also Japanese, Korean, Chinese, Filipino, Vietnamese). By public transport you can take the Northern Line to Colindale, then walk, or take Bus #32 from Kilburn. If you are driving they have free parking. They also have a great food court next to the grocery store so go hungry.
I used to go to the one on Chepstow Road (W11) fairly frequently and they have a good range of Thai groceries and some ready made preparations. There is also a small Thai grocery store attaching to Taste of Siam restaurant on Camden High Street. And a place on Upper Richmond Road (East Putney), I forget the name.
Look out for Maesri brand curry pastes, they are very good.