In reply to MG:
Only if the assumption that next year you will make the same profit a you did last year is true.
Doesn't work for people whose income have up and downs, which means on your down year you will have to pay as if you were having an up year (so not only you already have a reduced income but you also have to find money to pay for tax you do not owe and will not owe) and which you only get back after your tax return (hence you have paid 'in advance') even if it's glaringly obvious your turnover isn't even close to it (you cant file a tax return yet for the year which is when your tax lliabilities are set, hence, you are paying 'in advance')
Pariculary machiavelian when you are already getting tax deducted via other means such as CIS, etc. So HMRC is already taking a 20% of your gross turnover at source, plus you have to pay another ~20% on top on that years income, which if you couple it with a down year it means not only you have a reduced income, but you have to give away >40% of it to HMRC until such a time you can file a self-assesement to claw some of that money back.
The whole scheme is messed up