@sharkattack I really like the used section on Rich Tone – My Tele came from there, and I check it daily for interesting stuff. Don’t tell them (or anyone else), but they appear to have Facebook prices with shop levels of service. The descriptions are always rubbish but the pictures are good.
That SG seems very keenly priced considering it comes with a hard case (worth £50 ish I’d guess, although can be a PITA to store depending on your setup). I’d nip down and have a play – if everything works and the pots aren’t crackly etc then it’s a safe buy.
What sort of guitar do you want though? Arguably an SG is not the most versatile option. But if it’s what you want, go for it!
I’m guessing you’ll know what neck feel you like – the SG is shorter scale, usually wider and bigger radius (flatter) than E.g. Fender necks. I’d see if you can nip down and compare it to the Player Strat they have (or the Vintera one) and one of the many Ibanezes. It’s a bit too Spinal Tap for my tastes and won’t be great for clean tones, but if you want to play AC/DC there’s no better option in that budget…