Being in the Philippines I forgot about the bank Holiday. Major congestion is likely anywhere but the worst places are from M54 junction on M6 to M5 also around the Bristol area. Depends again on what time of day you are travelling. The earlier the better
I used to regularly do the journey from Reading to Birkenhead: at best 3.25hrs, at worst 5.5hrs. You'll be going a slightly different route, but going further south than Reading so I'd allow 5+hrs minimum... esp. as it'll be the start of the BH w/e. As Davao says, the M54/M6/M5 can be a nightmare...