I’ll second @D_A_S’s recommendation for UAV Hub for training. I can’t say how they compare to others, but they were excellent for my GVC training.
Couple of things to think about before taking the plunge.
Firstly, It’s not cheap. The GVC training will cost you several £100s (UAV Hub is currently £500). You’re also going to have to pay the CAA £234 for the PDRA01 Operational Authorisation (OA). Your GVC certificate lasts five years, after which you’ll need to redo the training, exam and flight test (£500). Your OA only lasts a year, so the renewal fee (£217) is an annual expense (and it gets more expensive every year).
You need to keep your Ops Manual up to date (which means keeping on top of the rules and regulations). As Trevor says, there are services such as UAV Hub’s HUB SUB, who will do that for you for a fee (another £100-£200 a year), or you can do it yourself. Be warned though; if there’s a mistake in your Ops Manual the CAA will reject your OA application.
Also - and this sounds obvious, but is worth saying - if you’re flying under your OA then you’re flying under your OA. You’ve got to do the admin and you’ve got to adhere to the specific requirements of the OA.
You’ve mentioned your goal is to be able to fly over buildings and people. An OA certainly gives you more scope to do that - one of the big differences between it and an A2 CofC is that the 50m separation in the OA doesn’t specify ‘horizontally’ so, theoretically, you’re okay if you’re 51m above them. That said, the OA also says overflight of uninvolved people “must be kept to a minimum”, so it’s definitely not carte blanche to do as you like.
As @ximi says, don’t bother if what you want to do can be done under the Open category. You might even find the more cost-effective way to get where you want to go is to get yourself a sub-250g drone. By the time you’ve paid for a GVC and the OA you’re most of the way to the cost of a Mini 4 Pro.
That said, I’ve just renewed my OA so I obviously see the benefit (one being the 50m bubble, the other being that I’ve had more than one job which would’ve been impossible without it because of landowners’ stipulations). So if you want it then go for it. 