No worries about it all being on one ticket, but if you're on the ground in London for more than 24 hours (or at least, scheduled to be) you'd have to adjust the price to include the air passenger duty.

Good news about the landside transfer; of course, there's no reason why you can't either end the Avios ticket in LHR and travel on a separate booking or, even better in terms of access rules, change it to terminate in e.g., NCE (unless that's a dealbreaker for a 2-4-1 perhaps?) This would leave you with INV-LHR| possible stopover | LHR-CPT | main trip |CPT-LHR-NCE all on one ticket.