To match your s400 isn’t going to be cheap

Anything below a £1k is going to be pot luck, I’ve got a few budget bpups that are as accurate yes but I’ve had plenty that weren’t

New your best bet for an accurate bpup out the box is something from Kalibrgun, AGT, Airmaks, Edgun ranges, used these are around a £1k each, then there’s the Daystate pulsar, AA Galahad (can have cocking issues)
Or look for a sorted budget pup as they are a tinkerers dream, I’ve a few Karl’s and other Turkish bpups that are excellent now but all needed tic, like regulating and rebarreling with just two original barrels out of a dozen guns I’ve bought have stayed put, the rest have had either LW, CZ or Cometa barrels fitted

From personal experience I’d give FiX a wide berth especially as you want accuracy, didn’t think the hwpup was anything but heavy and every Chinese gun I’ve had has had issue and with zero parts available I’ve stop trying.

For me, if I had to buy a bpup and guarantee accuracy it would be a kalibrgun cricket or an AGT Vulcan