Perhaps you should see if there are medical appointments anywhere in your area, and if you can find availability you can use this to challenge DVLA.
Try this list, but not all of them are approved for DVLA HRO medical, it is actually a list of approved doctors for Maritime medicals but many of them do the DVLA ones So make sure you ask. They will want your M number which appears on letters from DVLA.
https://assets.publishing.service.g...roved_Doctors_in_UK_06_October_2020_v0.24.pdf
BUT, it may be that it is not the availability of getting the urine sample, but getting it analysed if labs are full up with doing tests related to Covid, so as well as asking if the doctor can take the sample, get confirmation that the sample can be analysed.......