I have looked up the figures for the Chester elephant complex from the ZooLex website. The indoor areas are well within the new guidelines as the main area is 1000m² (which can be subdivided) plus 3 separate pens are 165m², 275m² and 407m² - the largest being the bull pen.
However the main outdoor area is about 6500m² in total, including the bull's area on the bridge side (550m²). The 2 off-show yards on the car park side total 270m².
As already suggested, the zoo could build a new paddock in the part of the car park outside the bull pen: but this would need a major reconfiguration of the indoor area to allow the cows to access this new area through the current bull pen or through the site of the hornbill aviary and the off-show pen behind it (which would also reduce the length of the visitor's area).
Alternatively the zoo could expand the existing paddock by extending it to the south and the west. This might involve demolishing the gift shop and most of the entrance plaza and building new ones in the car park, plus rebuilding the bridge over Flag Lane to give the herd access to the paddocks used by the Indian rhinos and brow-antlered deer, which would need to be rehoused or rehomed.
Either solution would be a major project and would involve creating new barriers along the side of the bridle path of Flag Lane, which might raise objections.
Another possiblity to consider is building an entirely new elephant complex on a virgin site, perhaps north of the current lion enclosure, which might be the best option if current elephant house is unlikely to have much future past 2040.
Of course the fouth alternative would be giving up keeping elephants altogether - which would not be popular with zoo visitors (ironical understatement

).
I think it will be a difficult choice.