At present, you can only apply for an International Driving Permit at a UK post office which is of no help to you and the many other people in the same situation. However, someone can apply at a post office on your behalf if they have the relevant documents. That’s far from ideal but if you have a relative in the UK who could help you out it may solve the problem.

You can get an IDP from the Post Office (they cost £5.50) however, depending on what happens on the 29th of March, you may need different IDPs for different countries. If there is a deal, UK licence holders will be able to continue to drive in all EU and EEA countries using their UK driving licence.

In the event that there is a no deal, the Government says it 'will seek to put in place new arrangements for EU and EEA countries to recognise UK driving licences when people are visiting, for example on holiday or business trips.' However, until then you'll need an IDP. You are right that you will need a different IDP for France (1968 IDP) and Spain (1949 IDP).