That's a dangerous thing to do. It contains a battery that, when heated in an incinerator, could be a problem.
Abbott states that it should be disposed of as a battery...
The NHS website states that if there are no problems for 2 years, then it should be every 2 years
https://www.nhs.uk/conditions/diabetic-eye-screening/
Your diabetic nurse is taking absolute rubbish. The only people who could ever issue a tax exemption certificate would be HMRC, and as it's the manufacturers that pay the tax, it would be them that would claim any exemption.