Flyingdutchman you are looking at the fare for travel via London as opposed to the Cross Country route via Birmingham which is £416.00 anytime, but if you travel after 0930 then it comes down to £232.40.
Time wise it is the same, but do reserve a seat on Cross Country in First. Alternatively if you wish to go via London split your journey ticket wise rather than having a through ticket. If you know the dates and times you need to travel you can get some bargain First fares between London and Liverpool in the off peak.
If you don’t have an Oyster card buy a Bournemouth to Zone 1 return ticket which will cover you on the Underground. http://www.fgwtickets.co.uk is a good site for any fares, irrespective of operator, but check Virgin
Alternatively take the train to Southampton Airport and fly on Eastern Airways Southampton Liverpool service which runs twice a day. Hope that helps