I think Adz will approve the 12 week dosing, it was I believe based on part of their trial data and will hopefully be a way of boosting the effectiveness. I can't see any country restricting travel unduly for a dosing regime that is signed off by the manufacturer
The more worrying thing is the Pfizer vaccine - this is against manufacturers advice. It may or may not be effective, and Pfizer may or may not be willing to sign off on the revised dose regime.
If it isn't effective, then we have bigger probelms like how do we re-vaccinate the population given the extended dose. If it is effective, but not approved, and other countries don't recognise it, then a subset of the UK population will have continued restrictions due to improperly given vaccines. Where will redress lie - my relatives signed up to the authorised dose and had their interval doses changed, without their permission after one jab. If they cannot freely travel due to this unilateral change (while the rest of the population can), will they have claim for compensation against HMG. Will the government be able to buy tier points for them to retain status in the 6-12 months extra that they can't travel while this mess is sorted out? What about lost work and social opportunities they would have had, had the jab been administered as per mfg recommended regime.

The UK Gov has a lot to answer for, and unlike the vaccine manufacturers, has not got a liability waiver!