I’ve had a post office refuse to take a boxed bike due to the size, despite the parcel force website and booking saying they would. The parcel force depot weren’t happy about taking it either. I’ve sent frames fine, but beware sending a whole bike!
If you use parcels to go then you can send it via parcel force for much less than going to them directly. I sent a 25kg tow bar to the other end of the country for under £10. I was expecting to get knocked back but no, it all worked out okay.