Lost records have been a problem since time began. Sometimes the loss is genuine and records ruined in fire or floods etc., but sometimes the “loss” is a smokescreen because the business person thinks they can outsmart HMRC by claiming the records are lost.
HMRC has lots of information sources they can access in these situations, including digging into your bank accounts when the statements are lost (have you any idea how much the banks now charge for each statement HMRC ask for? between £5 and £25 each depending how far back they are needed), looking at your lifestyle to get a handle on how much money you spend, interest information, dividend information etc.
Without records, investigations become much easier for HMRC because you cannot provide any rebuttals to assumptions they make about your financial affairs and those investigations always end up with a much higher settlement than if all the records are available.
So… try hard not to lose them – they are worth their weight in gold – literally. Micro-fiche them, scan them, copy them, protect them as best you can so that if the originals do go AWOL for any reason – then it is no big deal at all