As late as 1978, as a student, I was able to get a license for several pounds of Gelignite (Hampshire police - but to use in Yorkshire!). Probably not anymore...
In the 70's I seem to remember that an immediate use licence let you have a maximum of eight pounds of either stick or plaster. We always went for the eight pounds.
Pretty easy to get a bit of bang in the 60s/70s. IIRC, you were even allowed to carry your 8lbs on public transport! We used to buy ours from Casswells - pretty cheap, again IIRC.
The short answer is "bought it from a shop" - from what I've been told it wasn't uncommon for hardware stores in mining/quarrying areas to sell it; presumably you'd have to show your licence at least once to get it but prior to the IRA bombing campaigns of the 1970s controls on getting bang were pretty relaxed!