I know nowt about Carlisle but I suspect there's not much in the station at that time - you'd be best to find a pub nearby (but pick wisely on a Friday night!).
There's left luggage in Euston but I wouldn't suggest hanging around there. You could instead wander over to St Pancras (short walk) and have some breakfast there e.g. at the Booking Office before taking the Thameslink to Gatwick. But I'd probably just head straight to Gatwick and give myself time to relax airside before the flight - suggest buying access to No. 1 Lounge if you don't have it from status/class of travel.