Your owl is either a Bengal Eagle Owl (most likely) or a European Eagle Owl. EEOs can be VERY large, much larger than the fella on your armchair, which is why I go for the Bengal Eagle Owl, also known as the Indian Eagle Owl or Rock Eagle Owl (Bubo bengalensis). Frankly he just looks like a Bengal.
These owls are very common in raptor centres (they're one of the half dozen or so species that you'll find in every centre) and it's most likely that he/she is an escapee. I haven't heard of feral individuals in this country. Established escapees tend to be the larger European Eagle Owl.
Captive owls are almost always fed day-old hen chicks, so that's what you'd best get for him. He'd be unfamiliar with rats. There is no reason why you shouldn't keep him going initially on say raw steak. He should come to accept rats, and they'll be much better for him. Also make sure to provide a water supply (in a heavy bowl that he won't knock over) as owls like drinking. He may try to bathe in it.
It's likely he hasn't moved far from wherever he is kept and your best move is to report the find to the police in the hope that the owners will report the loss. I would avoid the RSPCA as they have a very poor reputation for dealing with these birds. You could also look to see if there are any raptor centres near you and ask if they have lost an eagle owl (though make sure to say bengal).
You are very lucky as this is one of the most beautiful owls. I agree he looks quite young, and in your first pic he looks a bit wet! He also looks pretty tame. Has he flown yet?
You'll find he is very messy and you'll need to take steps to protect your carpets and furniture. Don't let him into the kitchen as he may settle on the stove, with possibly horrible consequences.
There are some photos of the various captive species of owl, including this one, on my page here:
http://www.godsownclay.com/OwlGallery/owlgallery03.html
Hope that helps.