If this is likely to be a way of life, then I suggest Ltd. – easy to set up (30 quid and a form to companies house). “xcracer contracting Ltd” or some such will do.
Ltd invoices client (it’s the Ltd’s money), then the director(s) agree payment of salaries and/or dividends as appropriate. After which it’s your money.
Otherwise sole trader might be an option. Get an accountant to advise you about your options (that advice would be a legitimate business expense).
Depending on sector, take a little care in choosing an umbrella company – some seem to promise the moon on a stick, but a years worth of work starts to sound similar to an employment-like relationship.
VAT kicks in at £85K, at which point your clients bill goes up by 20%. It may be preferable to voluntarily register from the outset (and certainly if your rate indicates this would be passed in the year). VAT Info
Good luck… I haven’t looked back!