If you are transiting i.e passing though heathrow but not actually going landside (i.e in to the uk) you will be ok.
I think some other ladies have brought x rays and some countries they have to get a test as part of the visa application i belive.
You will still have to get some paperwork processed if you do use heathrow as your enter britain landside point and bring x rays. Maybe others know what difference it makes if it is busy.
When my wife arrived at heathrow there was no wait so did not make much of a difference.
like Piamed writes schipol is a lovely airport pass though it a little less than the man but one of our companies HQ is near there so spend many a mornign or evening hanging around there.