The need to 'renew' the latest NHS COVID Pass letters is certainly strange. My first letter, issued by NHS England in June 2021 after my 2nd jab in May 2021, had no expiry date. There were no '2D barcodes' (QR codes) on the letters at that time, just an 'ordinary' barcode at the top, next to which it said "Your unique reference - Use this to confirm your NHS COVID Pass". I used this letter, quite satisfactorily, to travel to and from France in Sep-Oct 2021, more than 30 days after its issue.
After my booster jab in Nov 2021 I requested another NHS COVID Pass letter. When it arrived nothing had changed - only my first 2 jabs were shown, no expiry date, no QR codes.
My latest letter (for TRAVEL), requested before Xmas and dated 20 Dec, is now different. The barcode at the top of the letter is still there, but there are now '2D barcodes' for all 3 jabs, i.e. including the booster, and it states that the "2D barcodes expire on: 19 Jan 2022". Given that my next visit to France would likely be for at least a month, even if I obtained a re-issue of the letter just before leaving the UK the 2D barcodes will expire while I am away!