As you probably know, the current UK tax year ends on 5-April-2019. One of the key lessons I've learnt is that it's best to start doing your tax return well before the end of the tax year so that you can assess your gains/losses early and minimize your tax liability by adjusting your portfolio accordingly.
Two other key lessons I learnt were to: keep detailed records of your transactions as you go along (e.g. on a spreadsheet) and, if needed, get professional help early. This is particularly true where crypto trades are involved. If you need to do your Crypto tax reports (e.g. as an input into your UK tax return) or to handle the full Tax Return in US (including Crypto reports), a good company which specializes in this area is TokenTax and can be reached via the following link:
https://tokentax.co/?via=jas
What lessons have you learnt ?