HMRC (HM Revenue & Customs) formerly the Inland Revenue are increasingly applying effort to chase up people who have not sent in a UK tax return.
However, not all people need to send in a tax return. These people tend to be where:
Anyone else who has tax to pay, or a special circumstance to disclose, losses, non-residence etc and especially if you have already been sent a notice to submit a tax return all need to send a tax return to HMRC.
Failure to send in a tax return that has been requested will result in HMRC writing, applying penalties, estimating your income, your tax and applying further penalties and generally making a mountain of paperwork and nuisance. In extreme cases of delay HMRC will apply for daily penalties and then to the courts to seize assets.
My advice is always for you to prevent this from arising and take a qualified opinion of your situation as early as possible to ensure that there is scope to reassure you and to plan an approach to HMRC if necessary. Leaving things too long will antagonise HMRC and this allows them to increase any tax based penalties.