I'm not sure you'd see much benefit in spending £1,000 on a new groupset for an Allez.
What's it currently running? Googling suggests Tiagra & some other bits.
Personally I'd look at upgrading to a 105 group and saving my cash for a new frame, then transferring and upgrading as budget dictated.
As I mentioned this was just a thread about is it possible but I knew people wouldn't be able to resist :-)
Turning your logic around, why buy 105 when I'm anticipating getting a higher end frame set long term. May as well get all the components I want now, if I only upgrade to 105 in a year or so time I'll then just want to upgrade it again anyway. I know overall its usually cheaper to buy a whole new bike but this way would make it two smaller hits rather than one big outlay.
Plus, I've done some pretty long rides on my Allez, flat and hilly (including C2C in a day) and its very comfy and handles well. Part of deciding between my options will be to try my mates Tarmac with mechanical Ultegra to see how much better a carbon frame feels. If the answer is "a lot better" then I'll be going for a new bike and sacrifice electronic shifting (can't afford both straight away).
Maybe that explains some of the reasoning behind it being "an" option :-)