Check the fare conditions of what you have booked. If its a flexible fare then you could subject to availability, if not then there will either be a fare difference and change fee, or no change allowed.
100£ change fee and since this would be last moment there would most probably be a hefty fare difference.
I am looking for tricks of the trade of a freebie change (e.g. if the late flight is pretty full it makes sense for them to send me earlier)