It does seem that forces are at work in UK to restrict the sale of SN. Forgive me for repeating that the appointment of a "Minister for Suicide Prevention" ( a tabloidism if ever there was one) less than a year ago probably lies behind this, at least in part, by bringing extra pressure on suppliers, starting with the big reputable ones, and then moving on to the smaller sellers. The reasons given by suppliers for declining to provide are less than completely honest, to make refusal easier for them. You now increasingly see smaller quantities listed as "out of stock" or similar. Who wants to buy - or try to buy - 25 kg of the stuff, when our need is for no more than 25 gm ?
SN is a "reportable" poison rather than a regulated one, as I have explained before. However, the Poisons Act 1972 contains what we call a "Henry VIII clause" which would allow the government to shift substances between the reportable list and the regulated list without primary legislation. No doubt there is official reluctance to shift SN, as it would be restrictive to legitimate trade, given that a Home Office licence would be necessary to buy it. However, if more subtle measures don't work then the government could make the change by means of a Statutory Instrument (secondary legislation) which is a relatively straightforward thing to do.
Why do you think delivery from Poland to UK will take "weeks" ?