Oh my God! Imagine what it was like several years ago when none of their aircraft carried the doctor's kit!
Question - what would be the point of carrying a doctor's kit on aircraft with no HF?
Several years ago all airplanes were made of wood as well.
This doesn't mean we shouldn't go along and embrace the modern technology and safety measures.
But to answer your question the doctor's kit has nothing to do with HF. The kit can be used by medical professionals. Operators normally have an insurance to cover the liability of the medical professional while trying to save a pax their life. (I've left Jet2 and can't check if they have this insurance, but presume they have)
HF (fitted on some of the Jet2 classics and NG's) can be used for contacting MedLink (or MedAire in Jet2 their case) via Stockholm radio. They can only give advice what treatment should be given, support the medical professional/cabin crew and advise if a diversion is necessary, if it is they arrange the medical readiness at the diversion airport.
Carrying a defibrillator on board and having the crew trained in its use should be pretty much standard. Luckily most UK airlines do have one onboard.