Can I travel to the UK on my multiple visit visa if I intend to
receive medical treatment which will be fully paid for?
Yes. The controlling technical reference is the awkwardly named "Appendix 3 of Appendix V" (Permitted Activities List) and specifically Paragraph 26...
26 An individual may receive private medical treatment provided they
meet the additional eligibility requirements at V 4.14 – V 4.16.
So we turn to V 4.14 and see the restrictions, which I'll summarise...
- if you have a communicable disease, you will need additional
- you must have made prior arrangements for your treatment;
- you must present a letter from the UK doctor describing the length of
the treatment (6 - 11 months). And depending upon which country you
are from, a certificate that you are clear of tuberculosis
Does this need to disclosed to the immigration officer when I land in
Absolutely yes, it must be reported to the Immigration Officer who lands you because it is likely a change of circumstances from those extant when you received your 10 year entry clearance. You need to report it even if the IO does not ask (deception by silence is an immigration offence by the Immigration and Asylum Act 1999).
You will have to prove that your treatment has been arranged and produce the letter from your UK doctor. Additionally, the IO will certainly drill down heavily on your accommodation arrangements and how you plan to support yourself whilst receiving treatment in the UK.
If you have all those bases covered, you can expect a smooth transit through the control point (if slightly longer).