I would recommend that you contact HMRC or the personnel department of your employer for accurate advice. I am an accountant but wouldn't want to give you advice on this as it's not something I have experience of.

It doesn't matter that you'll only be here for 9 months - you will still have a tax liability because you will be earning more than the personal tax free allowances within the tax year 2018/9. If you are an employee of the company being paid a monthly salary they'll have to register you for PAYE and deduct tax at source each month as if you were here as a permanent resident. This will possibly mean that you will be entitled to a rebate of tax as you move back to France. There will also probably be an NI liability too but as Elaine says the Human Resources/Personnel department of the London operation of your company should be able to sort all this out for you.

On the other hand if they are paying you a fixed lump sum and your status is that of a self-employed sub-contractor then you will still have a tax and NI liability but you will be responsible for sorting out paying the tax direct to HMRC yourself. In that case you should contact them direct. You eill find a lot of very useful info on their website and as long as you file your tax return by the relevant date they will work out how much you need to pay.

https://www.gov.uk/topic/personal-tax/self-assessment

Surely it will count as a secondment from the French operation?? So, your salary should be paid thru the French payroll ?

The taxation thing is more complicated than people my think. The OP says £30k after tax - is that UK or France. Temporary accomodation? Who is paying that? Yes whilst working in the UK then you are liable for Income Tax and National Insurance however the UK and France have a double tax agreement in place for income and capital gains tax which limits liability to tax for some types of income and gains to just one country.
As a french resident temporary working in the UK then you should be covered by this. The smiple way to sort it out is toe amploy the setvices of an accountant, your London office should assist you in this.

Just to add when youngest was worked in Vancouver in 2015 his earnings were subject to the dual tax agreement and employed an accountant - it made life a lot easier and was not expensive.

Hi Matthew
Have any of these replies helped you as I see you have been on but not replied.
When do you make your move to the UK?

I was expecting the money I make in the UK to be taxed by HMRC - I had no idea about double tax agreements! I'm probably going to get an accountants help once I'm there and earning money again. Will they be able to sort out the dual tax thing or will that have to be done beforehand?

They've been super useful! A holiday forum was definitely a stretch but by far the most useful information I found here I'll be moving in the last week of this month and starting very early next month.

Just been thinking on this and the question arises where are you going to be paid? My youngest son now lives in Los Angeles but back 5/6 years ago when he was based in London on several occasions he went to the USA to work on jobs but was paid and reimbursed in the UK for any expenses such as accomodation. If your company are paying you in France then there would not be a tax liability. The best people to advise are your employers accountants, any expenses incurred by you they will claim back against taxes so will know the best way of doing things.

Fwh, much will depend on how long Matthew will be in London within any one tax year. The general rule is that even if being paid from the French company base, if he is resident in the UK for more than 90 days then he will almost certainly incur a UK tax liability. I have friends who were in the merchant marine, sailing on 'flagged out' ships for companies based in tax havens but if they spent more than 90 days in the UK within any one tax year then they lost their 'non- dom' status for that year. One friend came home early before the end of the tax year because his Dad was dianosed with a terminal illness, exceeded his 90 days in the UK and was whacked for tax on his entire earnings during that tax year. He considered it worth it because he was desperate to get back and see his Dad but one can never automatically assume that just because one is being paid in one domain that you are not liable for paying tax in the domain you are actually living in. Matthew really does need to check out was his tax status in the UK will be.