Switch to Universal Credit

I've had the letter (and reminders) telling me my working tax credits are stopping and that I need to claim Universal Credit. The original letter said I needed to claim by 21 November, but I had another last week that said I need to claim by 11 December. I don't intend to claim Universal Credit though. I currently have a Tax Credits Exemptions Certificate for NHS charges (the current one expires in April 2024) and I presume that this will expire when my tax credits officially end. I just need to know when my Tax Credits NHS exemption certificate will expire though as I'll need to buy a Prescription Prepayment Certificate. The NHS Business Services Authority told me I need to speak to HMRC though, but despite trying to get through to the HMRC helpline a number of times, after going through all the security questions it tells me that the service is too busy to take my call and to try another time! All I need to know is when my exemption certificate will expire - the original date I was given to apply for UC, the revised date? If anyone can offer any guidance on this I'd be most grateful.

Hi dizzyhedgehog,
Once tax credits ends the Tax Credits Exemptions Certificate for NHS will no longer be valid, however customer may be able to get help depending
on their income under NHS Low Income Scheme | NHS Low Income Scheme, check this link. 
Without sight of your claim and records I am unable to confirm the date tax credits will end. 
Thank you