We have a saying in England.
Man up rofl
...
3mm, high waist pants for deep diving, 4 degrees at the bottom, no gloves and get on with it.
....
3mm? If you were from Yorkshire, you'd probably consider even Speedos excessively warm & unmanly.
I use a 5mm spearo suit (i.e. high waist band rather than farmer-john/salopette style) - and I've used it every month of the year except Feb. & March (which tend to have awful weather & visibility). However, it seems folk used to warmer climes often feel the cold more.
I believe you can sometimes get 5.5/6/6.5/7mm suits too.
While you might assume a thicker suit (e.g. 7.5mm) will make your life more comfortable/easier, I would make 2 counterpoints:
1. it is possible to overheat. When I first got my wetsuit (i.e. maximum "loft") and used it for the first time in November it was incredibly warm, I was baking.
2. The thicker the neoprene, the more lead you will need to carry. I carry 22lb and that can be quite a burden when carrying all your gear & fish up hill.
The British south coast sea temperatures are allegedly moderated/warmed by "the Gulf Stream" - warmer water from the Gulf of Mexico. Although that is sometimes hard to believe
.
So, if you think you might need a bit more insulation than me, Podge's suggestion for long-John style pants would give you an additional (5mm?) layer of insulation around your torso/"core". Another option sometimes mentioned for those using spearo-style high-waist pants is optionally adding a 2/3mm neoprene vest underneath - that was my thinking when I bought my suit but I never found the need to buy a vest in the end.