Me and 3 others are driving up and staying over in Carlisle for a night (did that last year)
Looking at train times, I think the best thing to do is to book advance tickets from Kings Cross to Newcastle from National Express Train tickets for travel to London, York, Leeds, Newcastle, Edinburgh, Glasgow
(when they go on sale) and buy a CDR to Carlisle.
On the day, buy your Brighton- London tickets (3 for 2 and 4 for 2 offers apply). I think thats the cheapest way to do it
cheers