There are some very specific guidelines on how tax is handled for remote workers in the UK, it comes down to if they classify you as a UK resident or not and can be read in the RDR3: Statutory Residence Test (SRT)
Here are some key highlighted bits:
The test allows you to work out your residence status for a tax year. Each tax year is looked at separately, so you may be resident in the UK in one year but not the next, or vice versa.
If you’ve been in the UK for 183 or more days you’ll be a UK resident. There is no need to consider any other tests.
You’ll be resident in the UK for a tax year and at all times in that tax year if:
- you do not meet any of the automatic overseas tests
- you meet one of the automatic UK tests or the sufficient ties test
So, if you can guarantee you have been in the UK for at least 183 days of the tax year, you're fine to be considered taxable in the UK. If it's less, there are more tests you need to check to see if you are considered taxable or not.
If you spend less than 183 days in the UK, you need to check the overseas test. The first two are not applicable to you as you've been a UK resident for the last few years and I assuming you will spend more than 16 days in the UK for this year. The third test is a little more complicated:
You’ll be non-UK resident for the tax year if you work full-time overseas over the tax year and:
- you spend fewer than 91 days in the UK in the tax year
- the number of days on which you work for more than 3 hours in the UK is less than 31
- there is no significant break from your overseas work
A significant break is when at least 31 days go by and not one of those days is a day where you:
- work for more than 3 hours overseas
- would have worked for more than 3 hours overseas, but you did not do so because you were on annual leave, sick leave or parenting leave
If you have a significant break from overseas work you’ll not qualify for full-time work overseas.
- can apply to both employees and the self-employed
- does not apply to voluntary workers or workers with a job on board a vehicle, aircraft or ship
So it is possible to do 100 days in the UK and be classified as a taxable person in the UK (You can even do 91), but you need to make sure you do not fall into an automatic overseas as classified above. Once you have passed the overseas test, you then need to do the UK test. The first one is a repeat of the 183 day rule. The next two state:
You’ll be UK resident for the tax year if you have, or have had, a home in the UK for all or part of the year and the following all apply:
- there is or was at least one period of 91 consecutive days when you had a home in the UK
- at least 30 of these 91 days fall in the tax year when you have a home in the UK and you’ve been present in that home for at least 30 days at any time during the year
- at that time you had no overseas home, or if you had an overseas home, you were present in it for fewer than 30 days in the tax year
- If you have more than one home in the UK you should consider each of those homes separately to see if you meet the test. You need only meet this test in relation to one of your UK homes.
You’ll be UK resident for the tax year if all the following apply:
- you work full-time in the UK for any period of 365 days, which falls in the tax year
- more than 75% of the total number of days in the 365 day period when you do more than 3 hours work are days when you do more than 3 hours work in the UK
- at least one day which has to be both in the 365 day period and the tax year is a day on which you do more than 3 hours work in the UK
So, if you do less than 183 days, it gets more complicated and you should really speak to a tax advisor to be sure. But if you can do 183 days in the UK, then you're generally safe, from a tax reason, to be classified as a UK taxable.