I'd go for equalising CGT and income tax.
I'd abolish national insurance and put up income tax by an equivalent %.
I would stop the removal of the nil rate band but increase the income tax rate over that value.
I'd get rid of the triple lock on pensions.
You'd have to work out some way of stopping inheritance tax being optional (not literally but in the sense you can just tax plan your way out of it with a moderate amount of care) and then probably cut the % rate at which it's levied. At the moment it's so high above the thresholds that the incentive to avoid it is extraordinarily high.
Look at options for care provision for the elderly and disabled to become not for profit. Same for SEN schools and children's homes/fostering.
Completely reform the public procurement regulations which would in turn massively cut public expenditure due to making it easier to spend money in a sensible manner rather than being overburdened by regulations making it impossible to procure anything without it taking forever.
Fiddle round with the fiscal rules to allow preventative revenue expenditure to increase. At the moment we're stuck because preventative spend (e.g. obesity avoidance or early years interventions) pay off in 5-25 years, just like capital spending does. They reduce spend in the future but require a doubling up of spend now while you're still righting the historic failures to put preventative spending in place.
Change the random cliff edge extra car tax they brought in so it scales with car cost rather than being an abrupt edge that creates perverse behaviours.
Deal with the fact the planning regs focus solely on harms generated by the thing being planned instead of also accounting for the harms of failing to build that thing (i.e. that you'll get a housing crisis twenty years down the line...oh look?!).
And then once you've done that, I'd probably shift new build homes so they weren't VATable at 0% any more - currently house building firms reclaim VAT on all their costs and don't charge any on the houses.
Reform the VAT system as a whole because it's currently mind bogglingly complicated, especially in the public sector where there's a whole industry of paying public sector cash out to VAT advisors to help public sector organisations avoid paying more VAT back to the treasury (because this makes sense to each individual organisation and indeed is essential to enable them to maximise the good they do, but perversely leads to less money overall being used for the public good).
Council tax also needs reform, preferably levelling out country wide or even better pegging to local average wages.
Probably increase air passenger duty. I love being able to fly for £50 each way but ultimately it's mental and shouldn't be possible.
Those are the ones that I would do off the top of my head. I'm sure there are more in reality.
Actually I think the other one would be to cancel the student loans and switch them out for a graduate tax. Certainly for new students. Not sure what you'd do for current loan holders tbh, or indeed for students whose parents paid the fees at the time.