Let me have a go at suggesting why Brexit is not the principle cause, then.
The country voted to leave the EU in June 2016. It officially left on 31st January 2020. The "transition phase" then began. That phase ended on 31st December 2020 and the UK's final separation was complete. That was nine months ago.
Concentrating on the tanker drivers' problem specifically, two weeks ago (i.e. eight and a half months after our separation) there were no problems with fuel delivery. I would suggest that any tanker drivers from the EU (and industry figures suggest there are very few compared to HGV drivers in general) who planned to return home because of Brexit would have done so. They'd have had over five years to consider their position and there is nothing to suggest that many, if any, of them had upped sticks in the last fortnight. There are as many tanker deliveries being made now as have been made throughout the summer. The reason the "crisis" has developed is because, based on the fact that BP was suffering a few minor localised delivery problems, drivers were persuaded by the media that there was no fuel around. There is. It's simply that vehicles that would normally roam round until their tanks were nearly empty are now being kept filled to the brim and some drivers are filling dustbins and milk bottles with fuel.
I provided links yesterday in another thread that showed that the proportion of EU HGV drivers who have left the business between 2016 and 2020 was near enough the same as the proportion of UK drivers who have done so. Some of those EU drivers may have left because of Brexit some of them may have left for the same reasons as UK drivers have. The problem for the haulage industry is not Brexit. It is that hauliers have continued to provide poor pay and conditions for its drivers. This, coupled with the UK's continuation of the ridiculous "Certificate of Professional Competence" which originated form the EU and the reticence of the DVSA to get back to working normally means many HGV drivers have found employment elsewhere. This will continue until the management of these companies accept this and do something about it.
Ironically it is our membership of the EU and the freedom of movement (low pay and the CPC requirement) that has contributed heavily to the haulage industry's troubles. Anyway, the tanker driver problem will fade into insignificance because it seems workers at the Ellesmere Port refinery are soon likely to go on strike:
https://www.cheshire-live.co.uk/news/chester-cheshire-news/essar-oil-staff-stanlow-refinery-21720712
There should be more than enough drivers to cart around the much reduced fuel that will be available as a result.