Panpot,
well pleased and hope visit goes well. you're very lucky.
as for recipe i'd go for your fav dish rather than base. i feel having tasted real BIR base the bases on this site are well capable of delivering BIR curry.
for me the hardest to crack appears to be madras. the closest recipe i've found is the rajver
https://curry-recipes.co.uk/curry/index.php/topic,1454.msg12717.html#msg12717
. with the onion paste i feel i'm very close. i don't use lemon juice and make a few tweaks but which are very small personal preferences (madras must be sweet & tomatoey). i intend putting some passata into the next go following the advice given in the onion paste post.
for me madras is crucial as it's essential tool for deriving the other recipes.
my recipe for info is per 200ml portion: 1 to 2 tbsp fine chopped garlic, 2 tbsp reclaim oil, 2 tbsp tom puree, 1 tsp mix powder (spice mix), 1 tsp rajah curry powder, 1/4 tsp salt, 1/4 tsp chilli powder, 300ml base, 1 tbsp chopped coriander, splash worcester sauce, 1 tbsp onion paste.