As everyone else has said there are actully more than a few species of aquatic or semi aquatic moss and liverworts native to the uk that can grow in aquariums. The biggest limiting factor will be temperature as many will struggle with temps that are too high. In my experience it tends to be the case that our native species like cool, very well oxygenated water but several will do fine in an aquarium with temperatures up to about 25 degrees.
Some commonly available species such as Riccardia chamedryfolia, Riccia fluitans, Fontinalis anypretica and Fissidens fontanus actually grow wild in the uk but there are many more that you might succeed with. In my experience you'll have better luck restricting your search to clear, flowing streams rather than ponds and lakes but if you're anything like me you'll be grabbing a field microscope and plunging into every body of water you find in no time.
here's a list of a few aquatic species from the UK that will grow totally submerged. I can post more when i have time.
Fissidens crassipes
Brachythecium rivulare
Nardia compressa
Racomitrium aciculare
Jungermannia exsertifolia
Hygrohypnum luridum
Scapania undulata
Amblystegium riparium
Rhynchostegium riparioides
If you get out there and get knee deep in a few bodies of wild water you're sure to find a few. Best tactic is to wait for a decent dry period and then go have a rummage around the edge of rivers where normally the rocks would be sumberged. Also it can be difficult to exercise restraint but some of our aquatic bryophytes are critically endangered so best practice is to take only a very tiny portion from the wild and then grow it out yourself. When it comes to moss, if it's going to grow in your tank at all it will grow from a single sprig just as well as a 12" square.
Hope this is some help.