I'd simply use force of personality and if they try to wriggle mention that you are very active on these forums
I would absolutely not do this.
It is worth knowing that there is no such things as a non-refundale deposit, not legally anyway. If you want your money back, you get it.
I'd imagine that the price will have to remain the same. Really, they should have ordered the cab immediately, which would have therefore been at buy-in price that the price you were given was based upon. If the prices later changed and GuitarGuitar hadn't yet paid for the cab, that's just hardlines to them, not you.
I'd be firm if they want you to pay more and just refuse, say you want your deposit back and that you'll go elsewhere. GuitarGuitar are a struggling business that absolutely cannot afford to lose expensive sales, they're losing money swift enough as it is

They're not gonna let a sale of that cost walk out the door and lose £100 they've already banked into the bargain.