I had always thought it wasn't too much of an issue to move spawn. However Froglife - the main UK charity for frogs do view it as "risky". Others are stronger in their language saying never do it.
Basically there is a risk of transferring disease - a disease that is reducing frog populations called ranavirus - see: http://www.froglife.org/disease/ranavirus/. They have a map of its spread which shows the nearest hotspot to be Bristol. So instead of getting frogspawn in from another pond the recommendation is to just let them arrive - and indeed they seem to do just See more at: www.froglife.org/advice/FAQs/spawn_tadpoles/where.htm