Our house had a coal fired boiler, which we replaced with an ASHP in 2012. We got all the grants etc, but hate it. Despite filling cavity walls, upgrading rads and installing UFH to the 2018 extension, it just doesn’t do the job. 
Due to its age and changes to systems since our installation, any replacement would require practically a full new installation, so we are thinking of changing to oil. Unfortunately gas is not an option. Whilst we chose the ASHP to avoid volatile pricing and for eco reasons, we now just want reliable heating. It’ll probably cost less to swap to an oil system than upgrade our existing set up. 
Has anyone got experience of this/advice etc?

Hi.

Rads will be fine, and the cylinder similar, just the controls may need a re-work.

In terms of the running the UFH, particularly if you want to do this independently, you may need a buffer tank, or you could switch the unvented cylinder to a thermal store which will give you stonking hot water plus offer up hydraulic separation between the oil ( non modulating output ) and the low heat demand of the UFH.

Other than those considerations it's essentially an S-plan system boiler arrangement, so quite simple / straightforward tbh.