Looking a bit more into this it seems that whilst the 612 is a step up from the 525 I had it actually removes CA so the speed I was getting was going to be the most I could get from Three with B3 at 15mhz. The 612 does have gigabit ethernet so this isn't the bottleneck.
I managed to peak at 121mb down which is getting close to the ~95mb I was getting from B3 and then 28-30 I was getting from B20. I'm not sure if this is really enough to warrant spending the extra £100 to upgrade. Sure it gives me some future proofing and there is virtually zero chance of 5G coming to where I am in the next 5 years at a guess.
This morning it's flicking 4g+ for a split second then dropping back out so maybe it's more congested this morning.
I'm still paying for a standard plusnet line giving me 30/10 so this extra speed boost is negligible but at the same time bigger numbers make me happy
My connection history:
Plusnet = Waiting for connection
Three DL 65mb avg / 97.80mb peak UL 30.0mb avg / 32.9 peak
VM + 3 DL 400mb avg / 432.93mb peak UL 50.0mb avg / 76.82 peak
Virgin DL 380mb avg / 393.08mb peak UL 38.0mb avg / 38.31 peak