I’ve done London, and my girlfriend has done Brighton.
Why do you want to run a marathon? If you’re wanting to raise money, then there are lots of places available for the London Marathon if you’re prepared to raise about £2000. The atmosphere in London is probably better than any other marathon in the world, with the crowds being huge. It’s also probably the easiest to get to and move around for spectators too. London is flat and fast, but it can be very intense - if you like running in solitude it can be a bit overwhelming.
Brighton is a good atmosphere too, but it is hillier, can be very windy, and there is a long, bleak stretch where you run out to the power station and there is minimal support from spectators. I think that there are still general entry spots available too.
Either way, you’ve left yourself a decent amount of time to step up, as long as you don’t slack off until after Christmas. Start planning and working to a schedule now.
EDIT: Oh, and well done on the half marathon!