I recommend
http://www.techniice.com/2011/icepacks/models.html and freezer bags.
Intra-EU you will be fine with customs without declaration:
http://www.hmrc.gov.uk/customs/banned-restricted.htm#3
From outside of EU is another matter.
Does LHR even have separate customs for EU and non-EU? I seem to remember that at T4, past immigration, the flows get merged.