How far is your longest run to date?
How long have you been running for?
You have a couple of weeks to get back up to fitness, not that you should have lost much if any, just don't over do it this first week. You can still fit in some long runs and some easy running in between.
I don't see any reason why you should not be able to complete the marathon. Brighton is easy, it's quite flat - there is a small up bit at the start but you'll be feeling fresh, it's followed by a downhill to help you recover, then it's flat.
You need to plan your race stratergy so that you don't set off too fast then hit the wall. If you set off slowly and keep a fairly constant pace you should be fine.
Without knowing too much about injury - Hamstring often occurs when doing speedwork, so if you have been doing speed work, ditch that.