Has anyone had any experience of buying an el-cheapo domestic shuttle ticket, turning up very early and being able to get on the earlier flight?

My plans have changed for the Saturday of the Shamrock do and instead of flying on FR to LPL from SNN I will be doing SNN-LHR-EDI (best route possible) The SNN-LHR arrives at 1415 and there is a BA shuttle at 1505 leaving me with a 50 minute connection time which should be fine if things go to plan but due to having to ticket these separately don't want to risk it so am going to book the 1605 LHR-EDI.

If I turn up 40 mins before take off with my el cheapo ticket could/would they possibly get on the earlier shuttle? Obviously it is a ticketing thing but I have heard stories where it has happened before regardless of status?

Sorry for being so long winded, Happy Wednesday ^