Would your struggles at home be eligible for extenuating circumstances in regards to coursework? In that case, I don't see why that couldn't apply to a field trip.
Also, if you really can't avoid this field trip, is there anyway you could maybe attend, but in a way that would make you feel less anxious? I understand with anxiety, the thought of sharing accommodation with peers isn't the most appealing for you, so could you maybe organise your own accommodation in Wales, so then at least you'd have a place for yourself? Or is it in the middle of no where?