Alternatively if you prefer to do your tax planning at home, then I recommend SA2000
https://www.sa2000.co.uk It supports the main sections of Self Assessment. SA2000 is based on Microsoft Excel, so if you do not have that, then no-go.
Otherwise it replicates the paper input forms and allows the user (me!) to explore those "What-if" questions, that occur when looking at my tax affairs. Some things you can change over time, like the balance of taxable income vs growth investments. Some you can change at Tax return time, such as pulling back charitable donations made THIS tax year into the LAST tax year, whose return you are working upon. I once found transferring a £100 donation allowed me to avoid having to make payments in advance for the next tax year - but that is perhaps an extreme and unlikely opportunity.
SA2000 is accredited by HMRC, is free to download and use for whatever you like EXCEPT submitting the return to HMRC, for which you would need to buy a code. The current version available to download is for 2022-2023, the 2023-2024 version should be available a few days into the new tax year in April 2024. The internal workings are protected against examination by the curious.
but, that link is great, thanks