Using different size tyres on back and front axles .... is it detramental ??

Wow, I never expected to get such a varied response. Having said that, I'm still no nearer to a difinitive answer of will I do any damage, yes or no. At present, I'm enquiring about delivery from the UK, with quote, for two replacements. As previously noted, I certainly wont be driving back to the UK in one go, more like 500 miles a day with overnight stopover. At present, the most I drive at the moment in any one day is no more than 70 Kms on a round trip, and usually at a max of 65 to 70 mph, no more. I think judging by most of the response, I should be ok doing what I'm doing at present, but should do the change before I start the long trip. Are there any expert experts still out there who could add more info ??
Thanks again all.
DT