I spent 6 weeks in Oxford last year. It is true that, like in the US, many train stores are closing or going on-line only.
However, surprisingly most big department and toy stores still have some trains, mainly HO. The old line Department Store in Oxford, Boswell's had a separate train department on a different floor than the Toy Department.
The famed Department Store, Harrods in London has a great selection of trains. When I was there last summer, I saw HO, N, and some Basset-Lowe O tinplate. Prices were through the roof, however.
Near Oxford, Howes Models was closing its train department to mainly serve the RC hobbyists. They are still the importers of Heljan O-scale (2 rail - $1K per engine/$400/car) and do DCC installations on HO.
Didcot is a great place to visit. You can take the train from Paddington. The museum abuts the Didcot Parkway station and has an entrance from the Platform. Swindon is a couple of more stops.
You should look at a British Train Magazine in Barnes and Noble. Railway Modeller and Continental Modeller are the most popular. There is also Hornby Magazine.
When you get to the UK, stop at the first WHSmith and look at their magazines. There are several other magazines that are rarely exported to the US. All have ads and dealer lists.
What I found amazing is that living in an apartment in a older residential district of Oxford, I never saw any trains, even HO for sale at the numerous Yard/Tag sales on Saturday mornings. - I had expected to come back with some used, but well played with Hornby tinplate.
You'll see trains in the most unusual places. I stopped by the Lancaster Town museum because it had started to rain and I had 45 minutes before my bus came. There were several display cases of O gauge trains from the Lancaster area.
Enjoy the UK.
Lad