First, ParcelForce aren't in a position to build European, Asian and American super-hubs to build a worldwide network never mind starting a global airline - that's literally billions.
Second, if they can't build it from scratch who could they conceivably acquire? There's been a great deal of consolidation in the express parcels and freight business and there's only a few big players now. Are ParcelForce really going to buy TNT (market cap 6.7bn EUR) or UPS (market cap $53bn)?
Third, the business has changed a great deal over the last 10 years. The document business (easy and cheap to handle, not dutiable) has all but disappeared and been replaced by volumetrically inefficient, expensive to transport and difficult to handle parcel shipments with associated expensive and difficult IT solutions.
So that's why what was merely difficult but do-able 10 years ago for Deutsche Post is now totally impossible for Royal Mail. Being preserved in aspic as government owned enterprise with the bruvvas fighting automation every step of the way has completely doomed the organisation.