When and why did UK taijiquan practitioners start emphasising standing qigong?

1991.

Lam Kam Chuen's Way of Energy book was published and popular.

If you read material published before that time there is typically no mention of standing.

Lam wasn't even teaching taijiquan...
He was doing dachengquan/I chuan - a xingyiquan variant that utilises static postures rather than forms...

There is no static standing in taijiquan.