Hi,
I have a few ideas.
1) Class 37/5’s and 37/9’s.
2) LNER N2’s which would fit with your suburban coaches.
3) BR Banana vans including the Diagram 1/242 versions.
4) LMS Banana vans including the one that survives on the Bluebell.
https://www.bluebell-railway.co.uk/bluebell/pics/570027.html
Thanks Fred