It would have been far easier and fairer to give every adult an identical, set amount each month, or in a one-off payment to tide everyone over. Plus, it would have helped the economy because people with the lowest incomes would spend all of that money, paying VAT on some things, and also keeping businesses alive.
They couldn't have afforded to pay everyone the top end, and paying everyone a lower amount (e.g. putting everyone on minimum wage or even UC) would have meant a rash of private renters, for example, suddenly unable to pay rent or meet committed outgoings. In much of Europe this is why unemployment benefits start at a % of previous salary. It's not fair but unfortunately none of this is. However I do think it should make us reconsider UC, people who lost their jobs and had to wait weeks for a pittance in January were just as deserving as those of us losing jobs /furloughed now on far more generous terms.
In terms of boosting spending, I do think they should do a cash payment, perhaps - but not until lockdown is reduced. As it is, the cash would be spent primarily on the businesses that are already doing ok, like supermarkets or Amazon. Waiting until things are more open will mean that many of the businesses now shut down will share the benefit.