A PP said about the 30 hours, it's 21 if you are a lone parent. I took on a 2nd job to help.
That's not correct. For a joint tax credit claim there must be 24 hours work done between the claimants. For a lone parent it's 16 hours.
The 30 hours is something else, this is a bonus introduced in about 1998 to encourage people to do more hours. Essentially tax credits are withdrawn at 41%, on top of NI & tax, which can work out as a marginal rate of about 73% for some people, but if you work 30 hours, then a £825 bonus is paid, which offsets some of that.
For Universal Credit you are expected to work 25 hours per week if your child is 5-12, and 35 hours per week if your child is 13 or over. These aren't hard limits in that you are supposed to 'look for work'.
There is no 30 hour element for UC because the government's idea with Universal Credit is that you should work more hours whether it's financially worthwhile or not.
The UC withdrawal rate is post-tax, and is 63%, which given a legal minimum 5% pension contribution, works out at (1-5%)-((((1-5%)(1-20%))-(12%))(1-63%)) = 71.32% 'tax', compared to tax credits which are (1-5%)-((((1-5%)*(1-20%-42%))-(12%))) = 70.9%, on income over the personal allowance of £12,500