Can't you get married and apply for immigration to Britain at the age of 40?

Yes As long as other immigration conditions are met, you can immigrate to the UK.

The applicant is over 18 years old;

The applicant and the guarantor have been together for at least 2 years or have a de facto partnership, and plan to get married within 6 months after entering the UK;

The English level of the applicant must reach CEFRA 1 (IELTS Life Skills Test A 1).

The applicant has no criminal record;

The applicant is in good health and has passed the tuberculosis test.