At the moment I have an Epiphone dot (semi acoustic, about £300) and a Vox AD30VT (about £140)
I did read the "read first" thread, but I'm still not sure if I should go for a new amp or a new guitar next.

I play a lot of Muse, and a range of other modern rock, Foo Fighters, bit of RATM, some Feeder, I would use my epiphone for acoustical stuff if I got a new guitar, so a new guitar should be more suited for fuzzy tones, but not a stranger to lighter stuff. I'm thinking HSS.
A new guitar would be better because a hollow body isn't really perfect for the style I'm playing right now.

BUT if I should get an amp, would it be wise to get just a cab instead of a combo amp. I have a Boss GT8 at the moment, so my Vox AD30VT is just set to clean, all knobs more or less in the middle, don't tend to touch it much, so I don't know if I need an amp head or not.

Thanks for any help.
Any suggestions of guitar brands/models or amps would be appreciated.
What is your budget?
What kind of sound do you want?

You should definitely get a new amp instead of guitar. Many people try to buy more and more guitars to get that sound they want when it is the amp that determines most of the sound.
unless you know you will be gigging in the near future, i personally would go for a guitar, if you have a band and such, i guess the amp might be the better way to go
Jackson DKMG Dinky (EMG 81/85)
Ibanez GIO (i put a Dimbucker in the bridge)
Crate GT65 (65 watts) to be upgraded soon, suggestions welcome (must be tubed)
Floor Pod (for sale)
My budget would be about £400-1000, but I don't need anything particularly loud as I mostly just play in my room, but if I ever do join a band it would be helpful to have a good amp. 50W should be plenty. (I play fairly loudly in my room, 30W suffices me, 15W is waaaay to quiet)

As for tone, Muse-y type tones, fuzz distortion 'n' such. I think Matt Bellamy used/uses Diezel amps, which I don't expect to get, but I guess that sort of tone.
Amp. With a good amp, Epi Dots dont sound too bad. For $400, you can get a Peavey ValveKing. I have a semi-hollow guitar as well, and can get a good Rage tone, a good Foo-ish tone, and can get a really nice clean tone as well. And the drive sounds much better than w/e is on your GT-8. And either way, you would need a power amp and maybe a preamp to play thru a cab w/ your GT-8.
get some sort of telecaster for muse stuff it works pretty well maybe a MIM telecaster or if you wanna spend a bit more go for the american standard model plus with telecaster head stock can do them picking behind the nut harnmonics matt bellamy does at (2.13-2-16) and (3.23) in that video of microcuts below