Could you just supply 5ml plastic ampoules?
There are problems with supplying 5ml plastic ampoules.

Firstly, they are very likely to be shared. Our experience of asking injectors to rate the relative risks of different water sources has shown that people frequently rate the risks of shared ampoules lower than those of other water sources that would typically have higher risks of bacterial contamination, but lower risks of viral contamination.

Secondly, the change to the medicines act to allow the supply of water for injections to injecting drug users only permitted the supply of sizes up to 2ml, and it therefore remains an offence to supply ampoules of more than 2ml in size.