Hi Stef 73,
Other post content which indicates that health beyond diet should be considered is correct. An underweight issue could be attributable to a number of factors. I would also comment that plenty of dogs maintain weight on what you have been feeding.
Whilst other factors need to be kept in mind I would personally switch to a different kibble. You correctly indicate that there is a relatively wide choice of dried Kibble at Pets at Home. If you go to their website you can and select dog foods and specifically dry dog foods. You can then do product searches on here using the directory.
The Wainwrights grain free looks o.k. on paper, (apologies for the Love Island reference), & I know others have fed it without any issues. Different protein sources can suit some dogs better than others... a quick check on PAH website shows 6 different types of grain free Wainwrights dried kibble....basically turkey lamb and salmon are all quite different. I would probably go for salmon or fish as may cause less gas output issues on brand change. I prior considered feeding this to my Lab. Plenty of other foods that they sell that rate similarly or better on here. If you want to keep it simple then buy a smaller bag and trial it for a few days. If I was looking at other dried dog foods they sell I might consider fishmongers (a PAH brand) or Fish4dogs to name but 2.
Cost is not always an indication of quality, however, it is fair to say that you cheapest prices often relate to foods of relatively inferior composition quality. Do be careful, however, as there are some big name foods at premium prices in attractive packaging that many of us would not feed.
Essentially you have come to the right place but please do your homework.
There is info on here re introducing a change in kibble....essentially you should ideally phase any new kibble in gradually. Bear in mind with better quality products you often feed a little less in the bowl per feed/day. As with any dried food ensure it is served in plenty of water....if feeding a small faced brachycephalic dog that has issues with immersed kibble then you will want to make sure dog takes in enough water via the water bowl, (from where dog is less likely to rush/gulp and inhale liquid).
Keep an eye on dog, look for obvious signs such as apparent discomfort & keep an eye on output. Expect some loose back end moments with your dog in early days of change and perhaps ease back feed amount if it continues.....adjusting as you go based on output and body condition. We have a Pug here & I like to feel rearmost ribs without any difficulty...perhaps seeing a faint outline at certain movement angles....Pug here can walk for hours & has muscle tone.....I appreciate condition varies based on diet, feed amount, exercise & age etc.
Good Luck & please keep us updated.