Re the lots of companies thing. HMRC have actually clamped down on this now and there is scope to make directors personally liable for company debts, but this is only a fairly recent change.
So previously, a company can set up and trade at very small cost. While trading, it can quite happily pay some or all of its bills. If it's a personal service company (ie a business that makes its money by charging an hourly or fixed rate for an individual to perform a service involving their personal skillset - for example a lawyer, accountant, or even bricklayer) the overheads and bills are likely to be small anyway.
So lets imagine that a company "X Legal Ltd" is set up, with a single director and shareholder. It provides legal advice, given by that person, who may or may not hold a legal qualification. It charges say £100/hour for that service. Over the course of a year that person might be spending an average working week of 40 hours providing that service pretty much constantly. So that business receives payments form clients of in the region of £200k in the first 12 months. Overheads? Maybe an office or a PO box, perhaps a landline and mobile, possibly a vehicle. The director should also be being paid a wage, HMRC has fairly strict rules about these types of company, but if it's a new business they won't know that these rules should be applied. So if the director pays themselves a wage of under the tax and NI thresholds there's no PAYE liability. So lets say expenses of £50k. being generous.
OK so now we have a company with a profit of £150k. This should be declared in the accounts and taxed under the corporation tax rules, but the company ceases to exist before the filing date then there is nothing anyone can do. A struck-off company can't be chased to file accounts or pay tax, it can't be fined, cos it doesn't exist. HMRC, if they suspect the company might be going to do this, can apply to have the strike-off held whilst they investigate, but that will only happen once a discernible pattern of behaviour emerges. So if one or two companies have say a different director, or a different trading activity, they could stay under the radar for a good while.
Going back to our hypothetical "X Legal Ltd". if that £150k progit is in a bank account in the company name, the bank will take it as it belongs to the Crown once the company is struck off and the account has to be closed. If however that £150k is elsewhere, it can't be touched because nobody knows it actually exists.
So that's one reason why a person might start a number of companies, and have them struck off before they have to file accounts.