Well... Half term is 30th May. A visit by Obama will have precisely zero effect on UK rail fares - honestly!

There's a major timetable change on 22nd May, and details are still being finalised, so very few seats have been released for dates after that. They're generally release on Friday evenings in bulk, so try again then.

If that fails, there are ways of getting there very cheaply (using slow London Midland services to Crewe) but they're much slower.