With a one-man company, you can choose to put allowable expenses either
through the company or on your own tax return as a deduction.

E.g., travel to a temporary workplace.

Imagine you spend £2,000 in one year on travel.

If the (your) company reimburses this, then the company saves 19% *
£2000 = £380 in corporation tax.

This also reduces the profits, so if the company would have paid that
£2,000 to you as a higher-rate tax payer, you would paid £2000 * 25%
= £500 income tax.

So you save £380 or £880 in tax this way.

If you instead claim the deduction on your personal tax return, then:

salary, £5045, dividend £18k - £5,045 is reduced to £3,045 by
expense, no tax is payable anyway, no saving is made. Dividends get no
tax relief, no money saved.
salary, £5045, dividend £72k. Deemed income is £85,045, and £83,045
after deducting the expense. Tax saving is 22.5% * £2000 = £450
salary, £7195, dividend £18k = £2000 @ 10% = £200 tax saving
£10k salary, dividend £18k = £2000 @ 22% = £440 tax saving
£10k salary, dividend £72k. Has lower actual earned income so saves
22% * £2000 = £440 and saves £2000 @ 22.5% = £450 because dividend
extends less far into higher rate bracket as well. Total saving £890.

This seems to confirm what is intuitive: if the expense saves you money
at a higher tax rate than 19% then it's better off on your self
assessment tax return, otherwise it's better off on the company tax
return.

So if you pay yourself £8,000 a year (£805 over the 10% limit), then
the first £805 of allowable expenses going on your personal tax return
will save more tax than having the company pay them. While if your
salary paid by your company is less than £7,195, then it makes sense
to get your company to reimburse you for all expenses, rather than
paying them yourself and claiming them on your tax return.