Are recruitment agencies allowed to force you to use umbrella co? I didn't think they were
Technically, they're probably not, but you might find they're less keen to find work for you if you don't co-operate on the umbrella front.
Make sure they aren't doing into make your hourly rate seem more attractive. It was (still is?) a common trick by agencies
That's DPs experience when he worked in the construction industry as a machine driver, which he has done on and off over the last few years.
Fortunately, that he has done in the past couple of years has been via the contstruction industry scheme, where they take a flat 20% tax off each pay packet, plus £10/20 pw fee. But he did get to keep just about most of his fairly decent hourly rate and the tax thing was all sorted out when he did his tax return.
Going back before that, he 'had' to use the umbrella company or else they were less keen to find him work and then what started off as £14 ph turned into NMW plus a baffling combination of expenses, bonuses with some tax, the umbrella fee and both employers and employees NI taken off.
In reality the money he earned was probably equivalent to about £10/11 ph and the worker and the government lost out while the umbrella company and maybe the agencies via commisions/kickbacks gained.
However, there has been a lot of pressure from construction industry unions and the norm now seems to be CIS, which is much fairer to the workers, who are generally machine drivers, brickies, plumbers etc who just want to earn a living.
Sadly direct employment by the construction companies, and employment rights, doesn't seem an option for many these days. Sounds like it could be the same in whatever industry the OP is in, whether it is construction, supply teaching or many of the other sectors that have gone this way.