You may be a touch out of date there Anne. Under UK law the relevant legislation is PACE S80
Commentary: A spouse or civil partner of a defendant is almost always considered a competent witness for either side, and may choose to testify for or against their spouse. A defendant may, when relevant, compel their spouse or civil partner to testify on their behalf. The prosecution however, may only compel the testimony of the defendant's spouse or civil partner in cases of domestic abuse or violence or sexual offences towards persons under 16. When the spouse or civil partner is a co-defendant to the charges, they may not be compelled to testify.
Note that while this gives special recognition to spousal relationships and civil partnerships, the law does not provide for a privilege and it is never possible to exclude evidence solely on the basis that it was a private conversation between a married couple.