If you are using a compliant umbrella company, there is no tax avoidance. It is a legal and reliable payroll option for temporary workers in a variety of scenarios, i.e. public sector work, not able to go PAYE or set up their own limited company, etc.
In response to whether you may be better off after Employers NI/umbrella company margin (usually a small weekly amount) is deducted, this depends on your personal circumstances. However, your agency should be providing you with an uplift in your rate to cover these deductions. Every compliant umbrella company will be transparent about this and will give you all the details you need in a personalised take home pay calculation based on your contract details.
It is common practice for recruitment agencies not to have their temporary workers or contractors on their own books, as it creates an additional administrative burden. Depending on your agency, you may not have a choice but to go umbrella as the agency may not enable temporary workers to go PAYE through them at all.
The umbrella company acts as the fee-payer, making all the relevant deductions that are required from the worker, once they are paid by your agency. The umbrella company will then pay your net salary into your personal bank account. There are dodgy umbrella companies out there such as offshore schemes, however if you do your research and choose a compliant and reliable solution, you will see that going umbrella is a perfectly viable alternative to PAYE.