There is no minimum working hours or income requirement as such. Paragraph 186(iv) and (v) of the Immigration Rules https://www.gov.uk/guidance/immigration-rules/immigration-rules-part-5-working-in-the-uk say that an applicant must show he/she:
(iv) is able to work and intends to take or seek employment in the United Kingdom; and
(v) will be able to maintain and accommodate himself and any dependants adequately without recourse to public funds.
For the applicant to meet requirement (iv) UKVI must be satisfied that the applicant either:
• has a job in the UK
• genuinely intends to look for a job (or become self-employed) and is realistically able to do this
It is highly unlikely that an income of £400 per month would be deemed sufficient to meet the maintenance and accommodation requirement. For comparison, the median average UK wage is currently around £28,000 before taxes, and the cost of living can be high depending on the location, for example see https://www.expatica.com/uk/about/basics/cost-of-living-in-the-uk-1167475/
Tax https://www.gov.uk/income-tax/find-out-if-you-need-to-pay-income-tax and NI https://www.gov.uk/national-insurance are not covered within Immigration requirements. Both are payable above specific income levels.