Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer
As this is your only client you should in fact be an employee and not self employed as per the employment status legislation.
However as en employee expenses can still be considered if deemed allowable. If there is no office in which you could work - then home costs would still be allowable, as would the costs to run the equipment such as telephone and internet and travel. If however there is an office from which you could work then you will need to discuss how the employer can manage you working from home as an allowable expense.
Whether employed or self employed he cost in tax and relevant National Insurance is about the same, so the main position which may affect your take home pay is whether the expenses are allowable.
So tax if employed OR self employed for this year allows the first £11,000 tax free then tax due after
With National Insurance things are slightly different between employment and self employed
As an employee the first £672 a month is free then the remainder liable to 12%
As a self employed individual ou have Class 2 National Insurance at £2.80 a week and then Class 4 with the first £8050 tax free and any reminder liable to 9%
So lets look at
1) Employment £25000 less £11,000 personal allowances = £14,000 x 20% tax = £2800
(This may be reduced with allowable expenses)
Class 1 National Insurance £25,000 less £8064 = £16936 x 12% = £2032.32
Total deductions £4832.32
Take Home per year £20167.68
2) Self Employed £2000 to £2300 (max) which is £24,000 to £27,600 a year but I will use £25,000 again to illustrate matters Income £25,000 less £11,000 personal allowances = £14,000 c 20% = £2800
National Insurance year of Class 2 £145.60 and Class 4 on £25,000 less £8060 = £16940 x 9% = £1524.60
Total deductions £4470.20
Take Home £20529.98
Reduced by allowable business expenses
So you can the added of benefit of sick and holiday pay leaves you only £400 worse a year of - and no costs to file self assessment tax returns or put money aside for self assessment (or payments in account) as deductions are made from your salary monthly
Let me know if I can assist further