It's perfectly reasonable that you don't want to them yourself - there is a bit more to them to changing them on a Morris Minor - there are individual coil packs to remove and some of them are awkward to get at.
My plugs were last changed by Abbey Motorsport as part of a P3 service. Abbey are highly regarded as an independent specialist.
The time quoted was 1.3 hours plus VAT - so the cost for the labour should be 1.3 X labour rate for the garage which will vary enormously depending on who they are and where they are.
Yes you can get cheap plugs - but it is not unusual to be paying £60-70 for the recommended plugs to be supplied by the fitting garage.
What you may be able to do is get the right plugs from someone like Zmanalex for a cheaper price and then get the right garage to fit them if they are happy to do so.