score:2
There is no 'renewals' procedure (that I am aware of!) so a fresh application is your only 'choice'.
It is possible (in exceptional circumstances) to apply to extend the duration of stay (see FLR(HRO) and FLR(IR)) but that is not the same as extending a visa.
Given that GOV.UK covers such exceptional circumstances I would expect them to cover the less exceptional "visa extension" were that possible, hence deduce that it is not.