Just got back from our new, re-vamped Morrison's. Don't know if there's a store in SE1, but I noticed they had fresh sage on sale for 99p a bunch. Also, all the other indigenous and some more exotic herbs and fruit/veg.
Lemon balm? Had a garden full of the stuff once and it took years to get rid of.
Been lucky with my home-grown herbs so far. Sage, mint, thyme, rosemary and basil all doing well and even the parsley has overwintered. Don't know if it will survive this so-called summer though!