The EU banned list prevents an airline from flying its aircraft in and out of EU airports (I'm not sure if this affects overflying rights).
In theory a UK travel agency could issue tickets validated for a banned carrier provided that the carrier participates in BSP-GB. However as the agent would be issuing tickets on the first international carrier, if that carrier were banned from flying to the UK, the situation is hypothetical.
A more realistic situation would arise if tickets were validated for the first international carrier, for example KLM, and a subsequent carrier was a banned one. The GDS would inhibit ticket issuance unless KLM had an interline ticket agreement with the banned carrier.
If the banned carrier did not participate in BSP-GB, and did not have an interline ticketing agreement with the first international carrier, the agency could issue an MPD/MCO to the banned carrier's UK office, or its General Sales Agent, and have them issue a separate ticket. The agency itself would not be able to issue the ticket and I'm not even sure if BSP would accept this scenario, in which case payment would have to be by cheque or CC.