Yes- the only people who make money out of an umbrella company is the umbrella company. In theory you may save a pound or two each week- in fact some guarantee to make up a shortfall if you are not a £ better off than you would otherwise have been) but even that can be smoke and mirrors.
If you submit valid claims for HMRC- approved types of expenses these are deducted from your income before tax and NI are due (so you pay less tax/NI) and then the expenses are added back. It can reduce your tax/NI bill considerably - eg if you reduce your taxable income of £500pw by £200 pw expenses your tax bill will be much lower.
But- you will also have to pay around 5% commission to the umbrella company- and you will have to pay employers' NI on behalf of teh umbrella company.
And you have to be able to produce full records of all journeys and expenses for an audit and maybe submit copies each week by post/fax/email.
Also, you may end up incurring costs you wouldn't incur just to get your tax bill down. My umbrella company encourages me to eat £10 lunches so I can claim £2 tax back, but really I'm happier with a 50p home made sandwich and a bottle of water which can't be claimed against tax