I think Sheila's 1827 baptism for Elizabeth Rebecca must be your Rebecca - the booklet I have of the Colnbrook PRs describes her father John as a thongmaker, which must have meant whipmaker - rather than maker of scanty pants!!

Is it possible that Rebecca's marriage to Ralph was registered under her 'proper name' of Elizabeth?
I've also found the burials of her parents in Colnbrook -
17 Nov 1870 Rebecca Jackson aged 79 of Colnbrook
Jill
24 Oct 1872 John Jackson aged 72 of Colnbrook