Did you buy anything yet?
£700 is a relatively good budget for a guitar and even more so in the second hand market.
Gibson SG's are ok, they are to an extent a 1 trick pony however and seem to have lots of horror stories - mostly down to user error however.
I like SG's but lots of people complain they are neck heavy due to the small body and it is a known problem of the necks breaking because of this - as i said before it is down to the careless of the owner typically.
Check out the Sterling range of guitars they are good for the money.
I think the Sterling AX20 would be a great guitar for you that can cover many many bases including AC/DC.
Amp wise, the Vox modelling amps are ok but it depends on how much of a purist you want to be for that AC/DC tone. A good way to get a similar tone is to get a low output valve amp. Such as a Orange Tiny Terror or a Blackstar.
However personally, you cant go wrong with a Marshall. It all depends on what you want! The used market is flooded with good amps and its opens so many more options. You could even find a 20w DSL combo that would do you fine.
These Sterling guitars seem to retail about £550 but I thought they were going to be a bit cheaper. £400 I think is the better market placement for them maybe they will drop. However that being said it is quite realistic you could pick up an Axis Supersport for shy of £700.
That would mean you're first electric guitar is perhaps arguably the best guitar ever designed