Jess is certainly one of the greatest British T&F athletes ever, but not even in top 5 IMO.
For me, the 2 most important criteria is an Olympic gold and world records. Ennis has 1 of the former and no WR. Next would probably be world ranking points and then 'other medals'. Obviously when comparing 2 athletes from last 25 years, then world championship medals are also important, just behind world records and Olympic titles, but if you're looking at all time, then you cannot use medals won in biannual global worlds, when they have only been available since 91. And they weren't available at all before 1983.
For all time Brits then Coe, Thompson, Edwards, Ovett and Gunnell, all won Olympic golds and set world records.
Coe and Thompson are top of that pile as they both won 2 Olympic golds and set more WR's than any of the others.
I'd put Ennis on the next level down, with the likes of Cram, Jackson, Farah.