In the US, I think the DGX Gears backplate is a no-brainer starting place:. $462 gets you a stainless backplate, wing, harness, crotch strap, and a pair of 4lb/2kg trim weight pockets. Since you are in the UK, the shipping could be high, I'm not sure. You can punch in your address and see:
DGX Custom - DGX Gears Singles Harness / Backplate / Wing Package
Other reputable brands I'm aware of: Halcyon, Apeks, OMS, DiveRite, Mares/XR,
What water temps do you dive in?
Things to look for, IMHO:
- Stainless plate if you ever dive in cold water, aluminum if you only ever dive warm water. Avoid plastic plates
- Single-piece harnesses are a good starting place, and many divers prefer them, I certainly do. I would not recommend starting with a harness with quick-disconnects or moveable parts. If you have issues with joint mobility, those quick-disconnects can be actually helpful, but for many divers they tend to add cost without adding much value
- Need a crotch strap. If your harness doesn't come with one, then they can be easily added onto the horizontal slot on the bottom of the plate. Wider crotch straps like 2" tend to be more comfortable than narrow ones, like 1"
- A pair of trim pockets are often helpful, even if you dive with a weight belt. They run $30 per pair or so.
- A single-tank adapter can make it easier to line your tank up correctly, and adds ballast. They run around $50 for a stainless steel one -- I've tried the soft ones, and do not recommend them
- Avoid any kind of padding or sleeves. These are upsells that try to solve a problem that does not really exist
- A wing with around 30 lbs / 15 kgs of lift is a good for warm or cold water. for example, my OMS 32lb wing works great in a T-shirt, 3mm wetsuit, 8mm wetsuit, or drysuit (all single-tank)
- Avoid wings that claim to be able to dive either single-tank or double-tank. It would be nice if a product like this actually exists, but it's a fantasy -- anyone who claims this is a liar, it will have big drawbacks
- Avoid plates that only take one size of wing, or only one brand of wing. A good plate should be able to take pretty much any wing, single-tank or double-tank, warm water or cold water. This lets you re-use the plate and harness for any kind of diving
