//Dealing firearms and class A drugs is not very different in sentencing.//
Not quite correct.
Whilst both offences carry a maximum sentence of Life imprisonment, the very lowest sentencing range for dealing in prohibited firearms has a starting point of six years custody, with a range of three to eight years. It is an indictable only offence meaning it can only be dealt with in the Crown Court. Sentencing guidelines are here:
https://www.sentencingcouncil.org.uk/offences/crown-court/item/firearms-transfer-and-manufacture/
By contrast, possession with intent to supply Class A drugs, at its least serious, has a starting point of eighteen months custody, with a suggested range of a high level community Community Order to three years custody. It is an "either way" offence meaning it can be dealt with either in the Crown Court or the Magistrates' Court. Sentencing guidelines are here:
https://www.sentencingcouncil.org.uk/offences/magistrates-court/item/supplying-or-offering-to-supply-a-controlled-drug-possession-of-a-controlled-drug-with-intent-to-supply-it-to-another/
In short, if a first time offender pleading guilty, is convicted of supplying a small amount of heroin to somebody and there are no aggravating factors, he can look forward to a bit of unpaid work, a curfew order, or a few appointments with his probation officer. If he supplies one gun to somebody he can look forward to a three year prison sentence.