48 weeks * 5 days a week is really best case scenario.
You have included 4 weeks of holiday, but not sickness, training courses, conferences, company admin, next contract sales and worst of all bench time between contracts.
My calculation are you are down to 45 weeks after 5 weeks holiday and UK bank holidays. Add 5 days for illness, 7 days for courses and conferences, 1 day a month for admin, 3 days every 6 months for sales, and suddenly you are down to 39 weeks. Add at best 2 weeks of bench time as lining up new contracts perfectly is tricky and you are down to 37 weeks.
£350 * 5 * 37 = 64,750 before tax.
Then it is the whole consideration of whether you are within IR35, which basically taxes you more than a normal employee, and is very difficult to not be within if freelancing on longer contracts in the UK.
Then accounting, insurance, hardware costs etc.
You would have to be on some real long term contracts without benchtime, no illness, no training and no expenses to get towards you numbers.
Working in Shoreditch myself I know most of the decent developers with 10 experience around here are earning a lot more than a contract guy would make on £350 per day.
Contracting doesn't pay as much as freelancing. Lots of people contract fulltime at companies and ignore the IR35 issues (even though they shouldn't) for around £350/day.
Freelancing is different. You have to deal with all of the issues you mention above, cost of sales etc (though, get yourself an accountant, seriously) but the reward is higher. You're doing things at your own pace and if you're good you can really focus to make solid profit.
Contracting is just a regular hourly wage (and as you rightly point out, not always as good as it looks on the tin).
You can use an umbrella company to do all your accounting. Ten minutes at the end of each week doing time sheets, 30 minutes each month submitted receipts for write offs. Equipment costs can be deducted from taxes so they can end up being extremely cheap or free.
IR35 is a good point. If you have multiple contracts and don't work in any one place for more than two years you should avoid any questions around if it's really a contract or not. Most employed coders I know hardly stick around in a place for more than two years here in London.
You have included 4 weeks of holiday, but not sickness, training courses, conferences, company admin, next contract sales and worst of all bench time between contracts.
My calculation are you are down to 45 weeks after 5 weeks holiday and UK bank holidays. Add 5 days for illness, 7 days for courses and conferences, 1 day a month for admin, 3 days every 6 months for sales, and suddenly you are down to 39 weeks. Add at best 2 weeks of bench time as lining up new contracts perfectly is tricky and you are down to 37 weeks.
£350 * 5 * 37 = 64,750 before tax.
Then it is the whole consideration of whether you are within IR35, which basically taxes you more than a normal employee, and is very difficult to not be within if freelancing on longer contracts in the UK.
Then accounting, insurance, hardware costs etc.
You would have to be on some real long term contracts without benchtime, no illness, no training and no expenses to get towards you numbers.
Working in Shoreditch myself I know most of the decent developers with 10 experience around here are earning a lot more than a contract guy would make on £350 per day.