Hi Houndlegs, the arriva train to Aberystwyth is my commuter train. I have never been refused entry with a bike by them, the train staff are usually actually very helpful. The only times i have had issues is when a previous train has been cancelled and i cant physically get on with the bike. Where are you joining the train and what sort of time?
Birmingham to wolverhampton can be very busy at peak times but anything west of Telford and things get much quieter. I did a similar thing last year but opposite, cycled to Aberystwyth and got train back with 4 friends all with bikes. Staff were a bit put out by this because we were a group but we still got home. Regularly 4 plus bikes during peak hours.
Just be aware that 4 carriages leave birmingham, but only 2 go to Aber, it splits at Shrewsbury.
MoN