... I assume that I can still start/end a RTW route from anywhere just so long as I purchase the ticket in the UK (at the UK price), or have I misread the rules?
I don't know, you should ask to a company or/and to a travel agency.
To come back to your last question: have you try to ask a travel agency based in England but with a desk/correspondant in Tunisia, for example, to benefit the Tunisian RTW fare?