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I don't really see what the problem is.

Whether we leave with a 'deal' or not does not impact data as we will still be a 'third country' to the EU. It's only if that deal specifically includes a clause that the EU will, using section 101 of EU Regulation 2016/679. The withdrawal agreement in Article 71 suggests some protections of personal data but does not state that the UK will be found to have equivalent data protections under this agreement. However having fully implemented GDPR then the European Commission could very quickly agree adequacy of data protection whether there is a deal or not - remember the USA is still deemed adequate despite being refer to the courts saying it sin't and obviously doesn't have the same safeguards as the UK.

Therefore accessing of data that is stored in the EU can still be access just by the UK determining that it is holds sufficient data protection when they formalise the Great Repeal Bill.

The issue then comes if the EU determine that they refuse to grant the UK a status that would ensure it is seen a adequate to protect EU data and they also feel that the data sat on the servers in the EU is now EU data due to residency and refuse to allow it to be processed by the UK. However how would they know if that data holds PII without somehow demanding to see that data.

I don't think anyone stopped using US servers when it was found that Safe Harbour was not adequate - I'm not sure why our GDPR protections and the EU GDPR protections would suddenly seem to be invalid and therefore the data storage location immediately relevant?