HMRC have numerous tools and calculators available on their website. Whether you want to get an estimate of the tax you may have to pay in the current year, your Statutory Maternity Pay entitlement or whether you want to find out if you qualify for tax credits there will be a useful calculator for you. The tools cover a wide variety of topics but the most recent one is about Preparing your Business for the UK leaving the EU.
There are 7 questions to answer about your business which leads you to a list of HMRC publications which will give you more information. If you skip all the questions there are 175 publications to access but if you answer the questions these will be reduced to only the ones relevant to the answers you have given. Here is the link to the HMRC tool Prepare your Business for the UK leaving the EU. There are too many topics to mention here especially as every business will need information relevant to their own business.
VAT will of course change if we leave the EU with no deal and there is a publication about how businesses would need to deal with that if they are selling to the EU or importing from the EU. HMRC have already sent letters to businesses suggesting the action they need to take if we do leave the EU and much of this needs to be prepared for in advance.
This is the link to the the list of other HMRC tools and calculators which are available.