1 You may be liable to CGT in the UK. The rules for calculating a gain are different in the UK from those used in France. Ultimately, whether you will have CGT to pay or not depends on the facts and figures of your case, in particular, the total period of ownership, the total period of occupation by you, the total period of letting, what the property cost to buy, the cost of any improvements and what it is sold for.
2 If the French property has been let, it cannot be your main home at the same time. As you own a property in London, unless you made an election within two years of having more than one home for one or other to be your main residence for CGT purposes, the determination of which is your main home will be based on the facts.
3 See 2 above. If it was accepted that the French property was your main home, then the London property potentially becomes exposed to CGT. You cannot have your cake and eat it I'm afraid.
4 There isn't much you can do other than sell it in a tax year when your incomes are low as the CGT you pay is determined by the level of your incomes. As the property has been let, you will be entitled to letting relief which can reduce the gain not covered by your occupation of it and the last 18 months of ownership of it by as much as £40,000 per part owner. See example 9 in HS283 for informtion on how this works. You use the exchange rates at the time of purchase and sale or the average rates for the tax year whichever is in your favour.
I hope this helps but let me know if you have any further questions.
Having done a calculation (months are more accurate than years), I have the fopllowing figures for each of you and your wife:
Exempt gain: £29,913 (£122,370 / 22.5 years x 5.5 years)
Non-exemt gain: £92,457 (122,370 / 22.5 x 17 years)
Letting relief: £29,913 (lesser of £40,000, £29,913 and £92,457)
Taxable gain: £62,454
Annual CGT exemption: £11,100
Net taxable gain: £51,444
CGT is charged at 18% or 28% or a combination of the two rates depending on the level of your income in the tax year of disposal.
I have to go out for a while but will be back a little later to answer any follow up questions you may have.
I'll get back to you on that.
You will be remitting capital, not income, from the sale of the French property to the UK. There are some notes on using non-UK income to service UK based loans only being treated as taxable remittances of income here and here. I cannot see how that can apply in your circumstances. The new rules came into being on 6 April 2014, long after you appear to have stopped funding the French mortgage from non-UK income.
If you wish to pay a fee, I would suggest you just pay a bonus as you posted the question here.