Universal Basic Income Coming To The UK

I still have deep reservations about the Universal Basic Income (UBI) but the radical social experiment which has been running in Finland for the past year is now coming to the UK with Fife in Scotland planning to use the Finnish model and paying their citizens up to £7000 a year regardless of their age or employment status by 2019.
The idea behind UBI is to scrap the welfare state and instead, pay every man, woman and child a monthly stipend whether they have a job or not, regardless of their personal wealth.
In Finland, 2000 people randomly chosen across all demographics have been receiving £495 per month and the trend, albeit from a very small sample, has been that people have reduced their working hours and are doing things more beneficial for society such as volunteering or starting up a business in the knowledge that they are still guaranteed an income.
My query is how could a country afford it but the answer appears to be by folding in all the current elements of the welfare state and the tax relief system.
The UK's Welfare bill is £253 billion, the population is around 60 million which breaks down to approximately £4216.66 per person or £351 per month.
The UK's Tax relief bill is £117 billion, again divided by 60 million Brits breaks down to approximately £1950 per person, or £162 per month.
By abandoning all tax reliefs and state benefits, each person in the UK could receive £513 per month and the UK economy would not be adversely affected.
Using these back of an envelope calculations it is plausible on paper but society is more complex and some will still require additional sums on top of the £513 a month stipend and that is where things get sticky and the bill potentially spirals upwards.
I like and support the idea that it will free people from their jobs and will contribute to society because you don't have to worry about making ends meet but the cost worries me if someone gets their figures wrong because the current austerity measures show the results when that happens and this will be so much bigger.