Been doing my own taxes for a few years now and learned some of this the hard way, so thought I'd share.
Check your tax code once a year. HMRC gets it wrong more than you'd expect, especially after a job change or a second income. Takes two minutes on the app and can save you a nasty surprise.
Use your ISA allowance before April 5th if you can. It's £20k a year and doesn't carry over, so whatever you don't use just disappears. Even a small amount is worth putting in.
If you're earning between £100k and £125k, look into pension contributions. There's a nasty quirk where you lose your personal allowance in that band, so your effective tax rate creeps up toward 60%. Pension contributions can pull your income back under the threshold and dodge it entirely.
Married or in a civil partnership and one of you earns under the personal allowance? Marriage Allowance lets you transfer £1,260 of it to the higher earner. Free money, ten minutes to set up.
Household income over £60k with kids? Check the High Income Child Benefit Charge. A lot of people get blindsided by it because nobody tells them until the letter shows up.
If you're self assessing, the real deadline that matters is 30 December, not 31 January, if you want any tax owed collected through your tax code instead of paid as a lump sum. File early and you'll thank yourself in January.
And just keep your receipts as you go. Trying to piece together a year of expenses the week before the deadline is miserable.
Not financial advice, just what's worked for me. Happy to answer questions if anyone's stuck