XC on trails (lightweight plastic skis) is not really a thing in the UK, which is why some of the other responses sound odd as they probably haven't heard of it. There are only two trail areas I'm aware of, both in the Scottish Highlands and not readily accessible without a car. Due to the infrequency and unpredictability of snow here, they are not usable all winter. Some XC skiers roller ski when there's no snow. There are groups you can google - be aware the standards vary widely. Serious XCers holiday in Norway, which is well set up for XC.

Nordic skiing on Nordic touring skis is of course possible almost anywhere as long as there's snow. There are many suitable Scottish Highland areas, little elsewhere unless you're lucky with the snow. The weather is the main obstacle, as often when there's good snow cover, there is also snowfall and winds. For safety reasons this is something I would only do in a group even on the rare crisp sunny day.