I live in the UK and in the disused gravel pits near me, which are now a nature reserve, there are freshwater oysters, which I have collected on occasion to eat, although with them not being farmed, they taste a little muddy.
I should think they could be found all over the UK in similar places.
You normally don't have to search for them because they tend to stay in the shallow parts at the sides of the lake, and last time I found 30 or so in a 10 minute picking.
I wouldn't suggest putting them in a tank unless it is going to be a species tank so you can observe them doing nothing. lol