The permit may not be the only "issue" they percive. You have to be 16 years old or above to buy livestock without a legal guardian acompanying you. It thus makes is a bit of a grey area as to if an under 16 year old
selling livestock is an issue... What would likely happen if you got a job there, is that you may be given the task of stocking shelves and selling the tanks + other dry goods, but not be allowed near enough to the livestock to sell any
It's also unlikely that you will be allowed to handle cash due to lax punishments being present to discurrage "minors" from stealing, so till work untill 16 would likely be out also... (we know you are unlikely to steal anything, but they don't)
You don't need to pay NI untill you are aged 16 or above
If the empoyer has their work permit, point out that you can legaly deal with dry goods, but understand that you may no legaly be able to do livestock. If they see you understand the legal corner they are in, they may be more inclined to find ways of working arround it to get you a job there. You know your hardwear suffice to botch stuff, so dealing with "off the shelf" gear shouldn't be an issue for you, and stocking shelves is far from rocket science
Finally, there is a limit to how many hours you are allowed to work in a week untill you are 16...
On a note of DIY though... Since you are going to have livestock in the tank, you want to fit a guard to the Juwel Phosphate removing pump's inlet
They can and will suck stock into them if left unguarded
All looking good for the moment otherwise though
All the best
Rabbut