Earning money while on leave

[Lakitu]

If a 6 month sabbatical was approved by my employer, woud l I be committing any kind of offence if I took another job elsewhere to earn enough to cover my mortgage payments?

[Twenty20]

In my view, a sabbatical is a break from work not a chance to work for a different company for a while. It does beg the question why youre taking a sabbatical if you need to work to cover your outgoings.

Doing charity work or traveling whilst on a sabbatical I would consider ok, but personally I would take a dim view of an employee going off to work for another company during this time.

[Lakitu]

I know it sounds odd LOL

I have the opportunity to move elsewhere in the UK, I'm just trying to cover the possibility of it not turning out as hoped and coming home.

I figured keeping my home vacant and taking a sabbatical is worth considering, but I also need to consider how I am going to keep up my mortgage payments and wondered about temping during that time. If all goes well then I sell my property here and hand in my notice officially, if it doesn't work out, then I am not stuck without a home or job.

[buildersmate]

That's not the purpose of a sabbatical. This obviously now follows on from your other question about whether it is legal to not return at the end of a sabbatical.
I don't believe that you are technically committing an offence. There would be complications with the P45 because your existing employer will (probably) not remove you as an employee, but you can get around that by filling in a P46 at your new place of work. There is nothing to prevent you having 2 jobs at once (even when one is paying you nothing). What happens with PAYE tax is anyone's guess. Usually when you have a second job, the second job is coded as BR meaning you pay 20% basic rate tax on all your income (or 40% if you earn into that threshold) - without the benefit of your coding allowance. If you try to unpick (to get the coding right) that there is an outside chance HMRC may unwittingly advise your existing employee about your second job. Of course at the end of the tax year it all gets adjusted so you don't lose out long term.
You perhaps need to think about whether you would ever want a reference from your existing employer - I would find a way of making reference to this in a reference request if I found that this had happened.