UK budget changes June 2010 - move from umbrella to startup own company?

Only you can answer that - are you going to be able to take on keeping the books up to date yourself, enough that you can hand them on to an accountant at the right price?

Basically, call up local accountants, and see what they will charge you. Actually incorporating is cheap (18 quid); whether you will need your accountant to provide company secretarial services is down to you. 5000 pounds is a lot to spend on basic accounting - it seems like it's at least worth you looking for some quotes.

Incorporate. Now.

Here's why:

1. It's massively more tax efficient. You pay yourself mostly through dividends, which are taxed at a lower rate than income tax and aren't subject to NI (0% to 37K then 32% above this point). You'd most likely pay yourself a small salary so the amount of NI you'd pay is minimal. You should end up paying about 25% tax.
2. You only pay corporation tax on your profits so your company's expenses (which reduce profits) also reduce your tax bill. Expenses include: IT equipment (laptop,printers, software), travel, training, membership to professional bodies, accountants fees, etc. I claimed about 7K worth of expenses last year.
3. If you do any outside work - I write mobile phone apps - it provides a great way to handle all the billing and money coming in from it.

Most of the admin can be made to go away by hiring a good accountant (I use SJD and they're very good - they cost about 1500 a year - they also set up my company). You do need to be careful about making sure you keep money back to pay your taxes - I've seen people get a tax bill of 30K and not have the money because they'd already spent it - so it's important to make a budget of what you're going to pay yourself and stick to it.