I'm also in a similar boat as you and was leaning towards Nvidia mainly because i'm already using an nvidia powered video card and Mobo. But I'd go for the 4770. It's newer tech than the GTS250 plus its cheaper. You can use the savings to add more RAM to your PC or add/upgrade some other parts.
The GTS250 is the better card, though I'm not sure if its 50 pounds better. (what is that, $100USD?) The GTS250 is the 9800GTX+, which is every so slightly better then the 4850, which is ever so slightly better then the 4770. I personally would buy the 4770, but I'm tight with money. (happens when your in my situation.)
I'm not sure what you mean by bigger better company, I am drawn to benchmarks. And they say the 4770 isn't that much slower then the 4850/GTS250.
The GTS250 is a little faster, but I don't feel that the extra performance justifies the extra cost. The only problem is finding the 4770 in stock, but it seems as though retailers are beginning to have some for more than an hour so demand is tapering off a bit . Just go to newegg and have it auto notify you when some are in stock.
Happy, check out scan.co.uk, ebuyer.co.uk and overclockers.co.uk too.
There are a (very) few 4770's available at slightly higher prices than release. I'm not sure that they are worth buying when they are priced higher than 4850's - you might be better served looking for a 4830 on ebay right now.