I can't help with northern London vets, I'm in the opposite direction !
Generally I buy premium cat foods, on the bases that it's still a fraction of my food costs, and I'd prefer not to feed junk to Zev.
The cheaper cat foods can have carbohydrates in them, which are cheap and bulk the cat food out, but cats are obligatory carnivores and naturally have very little carbohydrates in their diet, so it seems like a poor choice to me (not that we always keep to our natural diets very well!) Interestingly, cooked rice is generally considered to be fine in a cats diet, even though you'd think that contradicted the "obligatory carnivore" bit. A few of the cat foods I've bought have had cooked rice, and it normally gets enthusiastically demolished with the remainder of the meal.
I mostly keep to wet cat foods which are predominantly based on meat (Sainsbury's Encore seems good for that, and appears to be a cheaper "oem" version of Applaws). Schesir also seems good, but the only "easy" supplier for that seems to be
Zooplus, who are an online pet supplier, based in Germany. As said by others, most manufacturers seem to make specialised kitten food, which seems to be softer and easier for them to eat (and I suspect has differing dietary components, but I'm no expert on that).
I do tend to vary the brands I use, on the basis that I'd get bored eating the same food all the time, and to minimise the risk that any one manufacturer has something dodgy in their food. If I'm home, and Zev winges for lunch, I'll normally give her a little dry cat food, to shut her up, but her main two meals of the day are more traditional wet meat cat foods.
It's the same diet that Talisker and Kai have also eaten quite happily.