Boris Johnson says "I do", but in a very specific and limited way

[muriel_volestrangler]

10. In England, but not in Scotland

This absolute immunity lasted only until the entry into force of section 80 of the Police and Criminal Evidence Act 1984, which restored in limited cases the ability of the prosecution to compel the testimony of the spouse of the accused (later amended to include civil partners), namely where the defendant has been charged with "assault on, or injury or a threat of injury to" the spouse or a child under 16, or a sexual offence toward a child under 16.[9] In addition, under the 1984 Act, the defence can almost always compel the spouse to testify,[9] and as set out in section 53 of the Youth Justice and Criminal Evidence Act 1999 a spouse will generally be competent to offer testimony voluntarily.[11] However, a spouse (or civil partner) who is an active co-defendant to the charge can only testify for the defence (and cannot be compelled to do so by either side), part of her own right to the privilege against self-incrimination.
...
As of the Criminal Justice and Licensing (Scotland) Act 2010 spouses and civil partners are compellable witnesses.

https://en.wikipedia.org/wiki/Spousal_privilege#Testimonial_privilege_2

https://www.legislation.gov.uk/asp/2010/13/section/86