Booking it as separate tickets will cost more in miles than getting a straight-through single booking. But if there are no straight-through tickets available at normal redemption rates, buying separate sectors may be cheaper than the x2 rate.

It might also be worth looking at departures from other nearby airports to see if there are regular rate tickets available (eg. BHX instead of LHR).

In my experience, connecting QR to QR is not a problem for revenue to miles or miles to miles tickets, but I haven't tried it over the last few months since the oneworld changes. I have a revenue to miles connection coming up next month though so would be interested if anyone has up to date experience.

Edit - I see you are already considering other departure points. Given the higher UK taxes, you may effectively get your positioning flight for free by going ex-eu.