Hi from the UK too
I wonder if you're eating enough on your weekdays? Too little, and the body can panic and retain water ~ although that can often just be temporary and you get a bit of a whoosh.
I (a long-time veteran of most diets going
) have fairly recently started the 5:2 way of eating, which would work well with your weekends. Within that, I focus on protein, fat, fruit and veg, and do best by ditching the carbs such as rice, pasta, bread/cookies/cakes.
I'm losing weight fairly quickly but then, I have a whole lot to lose. The ethos of the way of eating, as presented in the facebook group I've joined for it, is that the weightloss might be slow for some people but it is very sustainable, so you'd end up feeling more like you were permanently maintaining rather than permanently dieting.
Final thought: they are very hot on telling us to measure as well as weigh. Scales can't tell the difference between fat, fluid retention (too many carbs) and muscle (gym work), to them it's just Weight; so they may stay the same/increase/move slowly when in fact you may have lost fat.
Not sure if I'm allowed to post links to the facebook group but it is facebook dot com/groups/the52diet