This is overkill, add 35% to the top and you will be closer (depending on absence/sick levels).
Statutory allowances in the UK are higher than in the US, and in the UK for instance you would add:
- 12.5% national insurance + apprentice levy
- 3-4% pension
- 10% of days as holidays
- 5% of days as sick / other leave (dependent on amount taken in reality)
- 2% payroll fees / other
Note, this excludes training and equipment/office space.