There is no reason why you cannot set up in advance, it's just that you'll have no Self employed income until you start working for yourself, but if you go on the payroll of the compay employing you as a consultant, you'll be classed as an employee, and as Boxtops says, they will be liable to account for Tax and NI deductions, although if you're over retirement age there won't be any NI deducted from your salary only Employers NI.
I found the best way was to set up as Self Employed and account for Tax and NI under Self Assessment, invoicing the companies I consult for on a monthly basis. This way my pension from my previous employers uses up all my Tax Allowance and I pay tax and NI at the correct rates on my consultancy earnings, after deductng allowable expenses.
See the following link regarding Self Employment:
http://www.hmrc.gov.uk/startingup/index.htm