Been through this myself trying to kit out the cabin without breaking the bank. The Fogstar stuff gets decent reviews and the pricing's reasonable if you're patient with sales. Their 5kWh packs are solid middle ground.
That said, honestly? I've gone with a mix approach. Grabbed two Victron SmartLiFe units on a Black Friday deal last year and I'm not regretting it. Yeah, they're pricier upfront but the BMS is bulletproof and the app integration saves headaches. If you're running everything remotely like I do, that peace of mind's worth something.
Renogy have some competitively priced options too, though you'll want to check the actual specs carefully — don't just go by the headline wattage. Their builds are decent but integration with existing setups can be a pain.
Real talk: most of the "budget" LiFePO4 out there perform similarly. What matters more is warranty support, BMS quality, and whether you can actually get spares in the UK if something goes wrong. Found that out the hard way with a cheaper Chinese brand.
What's your use case? Grid-tied, pure off-grid, or hybrid? And how much daily draw are you looking at? That'll actually determine best value better than just chasing the lowest per-kWh cost.