Nice choice! The K8 is currently my preferred board, although I almost gave it up. Just about six months into owning it, the LEDs began to flicker on one line of keys and then gave out entirely. Keychron offered me 10% of the cost of the board back, which I must admit, I felt slightly insulted by, given that I splashed out for the highest-end options. The RGB LEDs were a key feature behind why I chose this keyboard, and to lose a row is a big deal. To their credit, I let them know I wasn't really happy with that solution, so they did offer to refund the board. I will need to order a new one and then ship back the faulty one, with the original keycaps and switches included (admittedly a bit of a pain - I had customized the keycaps and switches, so I have a lot of switch and keycap pulling ahead of me). The shipping cost seemingly will not be reimbursed, but overall this seems a fairer option to me. Hopefully this was an isolated issue... we don't really have a ton of other options available to us.
For USB passthrough with a full-sized keyboard your best bet is probably something like a Das 4 Professional or a Das Professional S. They do not offer backlighting, though; the Das 5Q does have backlighting but I don't think it offers USB passthrough. Not sure about the UK layout bit though.
If you're OK with skipping the USB passthrough, something like the Keychron C2 is a full-sized keyboard that has RGB backlighting and also offers hot-swappable switches (which sounds like one of those superfluous features if you're new to mechanical keyboards but can actually save you a lot of money and heartache if the keyboard bug catches you... and six years and 6+ keyboards later, I can tell you from personal experience that you'll probably never see it coming until it's too late 😄).
Regarding switch type, I don't know that linear switches like the Cherry MX red are really all that different in sound from soft tactile like a Cherry MX brown. At work I have a mixture of Kailh Silent Rose (linear) and Kailh Silent Brown (soft tactile) and I honestly can't tell the difference in sound profile between the two, but there is a very clear difference in sound compared with the standard Kailh BOX Brown (silent tactile without the built-in dampening mechanism to quiet things down). Similarly, my home keyboard has a mix of Gateron Clear (linear - lightest switch on the market) and Gateron Brown (soft tactile) and I can't tell the difference based on sound. I'd choose your switch based on your preference for how it feels, and if you're new to it, the soft tactile is probably your best bet. I have a work colleague who went into mechanical keyboards and went straight into it with a linear switch, and he felt that it was overly sensitive and that he made a lot of typos with accidental keypresses as a result. He eventually got used to it, but soft tactile switches are a bit more of what most of us are used to.
And of course, if you do get a keyboard with a hot-swappable option, a lot of this is moot as you can fairly easily swap out your switches and try different ones for yourself. But if you go for an option like the Das ones, and you're stuck with the switch that comes with the board, be aware that there are O-ring silencers you can fit underneath keycaps to further silence a switch. I've tried those, too, and in practice I can say that they do quiet things, but unlike switches like the Silent Rose where the dampener is in the switch itself, you definitely feel key travel is affected. I didn't like the feel of typing with the O-rings installed, although I have read some people saying that they liked the feel of it.