It does depend on your overheads and what you need as a wage - even if your overheads are low you need to take into account tax, holidays, slow periods, equipment/software purchases and the inevitable bad-debtors (as well as legal costs if you're going to go for them when they try to screw you - try
http://www.thomashiggins.com as a solicitors letter before action costs £2 and usually gee's the debtor up to pay within 7 days but if they don't you're looking at £180 to take em to court which you'll get back when you win it, but you have to stump it up first); oh yeah, and build in a bit for profit too if you want to grow!
£200 a day will bring you in around £52,000 which sounds good but you should build all of the above in if you're serious; at that rate your wage may come in lower than you think.
I'd look at around £350 per day minimum (£50 per hour) - remember, you won't be working every hour generating income, you'll also be handling invoicing, phone calls and the obligatory hour or so at the pub for lunch.
Hope that helps & good luck!
P.S. Get yourself a bulletproof contract stating your terms of business and get it signed by every client - they're worth their weight in gold should they play silly beggars (been burnt before, but not again)...