Wetsuit recommendations

You might also overheat in 7mm wetsuit - I'm not kidding.
So, my 2cents: I too reckon 5mm - unless 2 or more of the following apply:
a. you are real skinny or small
b. you hail from warmer climes
c. you feel the cold more than most.

Bear in mind that...
The hood on a spearo suit makes a big difference, as does the stick-to-skin open-cell interior (provide you choose that option). Ditto wearing neoprene gloves (2mm super-stretch for Summer; thicker for Winter) & socks.

If you get the "Farmer John"/"long john" trousers/pants instead of the traditional high waist spearo pants:
a. it will cost more but
b. if you kayak, taking your jacket off, it will free up your arms while keeping your core/torso protected
c. it would provide 10mm of neoprene insulation for your core/torso (but see my first comment above).
Alternatively, you could just buy a thinner neoprene vest (e.g.2-3mm) to wear under your wetsuit. Eliossub offer one, although I recall that it seemed unnecessarily expensive - which seems odd as they could probably sell quite a lot at a more reasonable price - but perhaps the point is that you don't really need one! It is just extra junk that most folk can do without. But knowing that it is available, should your 5mm suit prove to be too cold (unlikely), it is reassuring that you could get a neoprene vest to solve that hypothetical problem. I think the Omersub stick-on rail works the same way : you can safely buy an Omersub speargun without a rail safe in the knowledge that - in the unlikely event that you really do need one - you can buy one later. Those clever Italians!

Pinniped72 is right about the extra lead: I wear a 5mm traditional spearo suit and need to carry about 22.5lb of lead for that ... I wouldn't want to be hauling more lead than that round on a walk-in.

BTW My suit was custom-made to my measurements (which I recommend) by Elios in Italy. But they have become very expensive (from the UK) , so I will likely get my next suit from Greece (esp. if they Grexit) or perhaps China if they start selling them.

Well, sometimes spearing can be quite exerting (e.g. in strong currents & when swimming to/from a dive spot) & you can get hot but there are other times when you are still for long periods too - and they do tend to be the chillier times. So it will be a compromise.

You sound like a hardy type though, surprised you even bother with a wetsuit, being from Yorkshire an' all
[I usually tell Americans that Yorkshire is like Texas and they seem to understand. As the bumper sticker said: "I'm from Texas, what country are you from?" ]

Actually you post prompted me to look into possibly buying a new wetsuit (or perhaps just pants) - there are sales on currently - and I find myself only considering 5mm (& one 5.5mm) wetsuit, even though 3mm & 7mm alternatives are plentiful. Actually I did once consider getting 3mm pants as I don't feel the cold there so much, it would help keep a cool crotch (as it should be for men) and it would stop my legs floating quite so much (some folk use ankle weights but after a while you kind of adapt to it).

If you are planning to dive up north (e.g. Yorkshire/Scotland), might be worth considering 6/7mm, either for the jacket or entire suit. But perhaps a 5mm wetsuit with 5mm long john pants would offer the best compromise. Of course, you'd need to carry more lead if you carry more insulation. :^/

Another option: buy a 5mm suit, you could always upgrade the jacket and/or pants later, once you figure out what work best for you. You can always keep the thinner gear for warmer days/locations and thicker stuff for colder locations/days. [ Gloves & socks help provide fine tweaking of warmth.]

This video makes some interesting observations & suggestions (e.g. cut the top off long jphn pants!):

More benefits of the vest option mentioned in an earlier post: