Can I swap out an Ultegra 6750 crankset with a 6700? I want to have the ability to change chainsets depending on terrain. I currently have the compact Ultegra 6750 chainset and am after a 6700 53/39 for the flats and races etc. Are the axels the same diameter and width? I have an S-works 2011 tarmac. Not sure what the BB is, I think it's a press fit BB30 but I could be wrong.

Any advice would be greatly appreciated. Thanks in advance.
The spindles should be the same width and diameter. Seeing as how you already have an Ultegra crankset, you should be able to use the larger Ultegra crankset without switching the BB or getting an adapter. But you will have to move the front derailleur up and probably re-cable it. Switching will make you have to constantly move it so you'll have to leave enough at the end when you're in compact so it'll fit when you move to standard.
If you're looking for more gearing but don't feel like switching everything back and forth, you could always switch to mid compact chainrings (52 x 36).  They use the same BCD as compact and it should be once and done without having to move your derailleur every time you switch.