In reply to mike123:
Hope you got my email Mike! I could split that bike if you only have need for bits of it.
For 24" forks the most promising 'off the shelf' forks I found were Spinners. Maybe a bit hard to track down but I did get proces direct from their European importer. The Grind Air 24 would be the one:
http://bluepill.pl/Spinner%20GRIND%20AIR%20LC%2024/9/13/
I did have a bit of back and forth with them about getting a 20" air fork made to fit an Islabike Beinn, as it would have needed a 1" steerer. They were communicative but it never happened.
The head tube dimension is the biggest issue with fitting any sort of non-spec fork to an Islabike Beinn as they're all 1" which massively limits what's available for them (unless you go seriously retro). I don't know what the Boardman frame you have has?
What I've seen a few people do is use a short travel 26" fork - which is tempting as there's a good choice of nice, light 26" air forks about secondhand (or in the shed). The risk obviously is that the axle to crown is really long and jacks the front of the bike up a bit. With a suitable sized headtube (1 1/8 or tapered) you could help this with an angle headset or slackset, but that's getting a bit faffy. With a short enough fork run with plenty of sag it may well be fine as it is.
You can get carbon 24" rigid forks (made for the BMX cruiser racing market I presume) which would be a good alternative - but again needs a 1 1/8 headtube:
https://www.carboncycles.cc/?p=197
Also if disk then you could run 24" wheel in a 26" fork - which you may have a chance of finding in 1".
On a 20" bike I just ended up going with as fat and light a tyre as I could find (2.25" Kenda Small Block 8) which can do a long way to making rough stuff more manageable. If you can bodge them tubeless then you'll be able to run lower pressures too.