Hi,
I don't know much of being a pensioner in SK, but everyboda who is coming to SK and intend to stay longer, or forever, have to get an stay-permit. To do this this person has to first find a place to register as permanent address (not necesserily address of living but do not mention it in the forreign office). He can for example register at his sons place, it costs almost nothing if anything. Then with a sort of of a "contract" he would need to show up at foreign police office, there is only one in bigger cities, must be one in Kosice. By doing so he will get sort of a Slovak ID and get rights as Slovak citizen with exceptions to vouting for government.
Once with the ID card he can purchase a house for cash, in my opinion with money for a house in UK he would be easily able to get a house in SK. And with UK pension he can have a ok life with no worries.
As to legal stuff regarding medical treatment and all that I can't help you, sorry. Getting morgage is also out of my knowledge.
I hope I helped ar least a bit.
All the best!