Which wetsuit thickness?

Hi

I'm fairly new to scuba diving (10 dives) and I'm not sure what type of
wetsuit to purchase. I will mainly dive during the summer in countries
where the surface temperature will be 30C or above (not too sure what the
water temperature will be) in areas likes the Mediterranean. If I get more
involved in diving I may start to dive during the summer in the UK where the
temperatures are much cooler.

What thickness of suit should I purchase? I've been considering a 5mm suit
but I thought this may be too much in the warmer waters.

Thanks

Ben