Hybrid or MTB recomendations

Without scouring the websites for deals and specs, the bike I'd put as the top contender to compare any others to sub ~£750 is the Voodoo Bizango. It's been a tried and tested bang for the buck front suspension mountain bike in recent years that reviewers often say performs as good as many £1k mountain bikes.

Suntour Raidon 32 LOR-DS fork
1x11 gearing (32T chainring with 11-42T cassette (approx 22.0-84.2 gear inches, very comparable to the 24/38T chainrings and 11-30 cassette {approx 23.2-100.0 GI} on my Voodoo according to https://www.sheldonbrown.com/gear-calc.html)
Hydraulic disc brakes
~13.7Kg as supplied
Sold as a "mens" bike but sizes start from 16", which they claim fits someone 5'5"+

£550 in current Halfords promo, if you ghet lucky you might 10% discount off that with British Cycling membership card) https://www.halfords.com/cycling/bikes/mountain-bikes/voodoo-bizango-29er-mens-mountain-bike-16-18-20-22-frames

Just be aware if your wife doesn't really need a suspension fork, which unless there are intentions of "serious" offroad can often be countered by a rigid carbon fork and handlebars with large volume tyres at low pressure (especially on a tubeless setup), there are considerable weight savings to be had. For example, https://www.halfords.com/cycling/bikes/hybrid-bikes/boardman-hyb-8-8-womens-hybrid-bike is suppled ~3.4Kg lighter than the Bizango at ~10.3Kg.
My Voodoo can weigh roughly as low as that Boardman when I have the road wheels fitted with "nice" 28mm tyres, which makes it far less of a grind up tarmac inclines than when I have the fat wheels fitted, mainly due to the extra ~4.5Kg+.