Family Action have a telephone line and can provide a listening ear, support & short-term telephone counselling: https://www.family-action.org.uk/what-we-do/children-families/familyline/#in-your-area
Care For The Family have a support line & will also signpost too, if required: https://www.careforthefamily.org.uk/family-life/careline
https://www.careforthefamily.org.uk/support-for-you/family-life/couple-support/
(Care for the Family also have a YouTube Channel too, including this clip all about PND in a relationship:)
https://www.youtube.com/watch?v=JSRqUuKIbTE
Marriage Care also offer relationship counselling (you don't need to be married, and it's not free, but essentially donations-based):
https://www.marriagecare.org.uk/how-we-help/relationship-counselling/
Talking Parents *Monthly fee*
TalkingParents strives to increase family harmony through better communication and transparency.
One Plus One
https://www.oneplusone.org.uk/separating-better