I really, really can't see the case for still allowing foreign school trips, or really encouraging foreign travel at all.
The vast majority of the 51 cases in the UK are people who've returned from other countries- but returning travelers will pass it on to other people. Most are not told to self isolate unless they are showing symptoms, by which time it will be too late.
If the government told people to avoid non-essential foreign travel, then people could claim on their insurance and not go.
As others have said, we are an island- surely we use that to our advantage?