Advice on when to weigh

An increase in weight when in a deficit, will not be muscle, but rather water retention. It is possible to build a small amount of muscle while in a deficit, but the scale would still go down, you just may gain a little muscle but lose more fat than the muscle you gain. (can't gain weight in a caloric deficit, other than water retention for a variety of reasons)

I also weigh every morning when I first wake up, after visiting the bathroom. Like pp said, daily fluctuations will happen but I find it helps me stay motivated to make good choices each day.

Frequency of weighing is very personal - my preference is daily.
I expect my weight to fluctuate from day to day, I understand why it does that (and it's not building muscle in such a short timescale!), it doesn't have any emotional impact on me - keeping an eye on my trend is my primary method of weight control.
From a data trending point of view less data points is worse, lengthening the time between weigh ins doesn't prevent those weigh ins being on a good day or a bad day and can give you a false understanding of progress made.

With 7 stone to lose realistically exercise is going to be a small part of creating a 343,00 calorie deficit over time (pounds to lose X 3,500 cals). Do it because it's good for your health and your fitness and gives you a higher daily calorie allowance to subtract your chosen deficit from. Also try to make it enjoyable which is one of the keys for sustainability both while losing and also while maintaining at goal weight.

My preference is daily with Happy Scale to give me trends and averages. Even though I knew better, it was challenging to see a 2# increase on "weigh in day" - I say I knew better because the increases were totally due to fluid and slow digestion.

I do weekly weigh in and ignore the scale at the rest of the week. However, this site suggest... "Taking your measurements can be an even better gauge of your progress because when you burn fat and build heavier muscle, your weight may not change or even increase even though your body is tighter and smaller. We suggest taking your measurements every 2-4 weeks".

I love the idea of exercise because it will make you feel better and, over time, more energized, and possibly increase your tdee thus allowing your deficit to come from a bigger pie (I'm a proponent of keeping deficits to no more than 25% of total expenditure even when obese, 20% when not)

Any single data point when it comes to weigh-ins is subject to error because of water weight variation.

The way you find the underlying trend is by increasing not decreasing the data points.

Happy Scale for iPhone, Libra for Android, trendweight.com and weightgrapher.com can all be used to help you see your weight trend over time.

I believe but I'm not sure that slimming world encourages you to change your way of eating towards what they consider to be more healthy options.

If this is the case that is a good start and basis to expand from.

While "free" foods may have been a great inducement to introduce some positive changes, very few if any foods are really free.

So now you have the opportunity to evaluate every item you're eating in terms of how many calories you're spending on it and in terms of the satiety and satisfaction it brings to you.

Your goal, of course, would be to maximize these while still meeting your weight loss goals!!!!

Stretch goals might also include things such as eating at least 0.35g of healthy fats per lb of goal weight in the "normal weight" range, 0.6g to 0.8g of protein per same, a good 26g or more of fiber per day, your 5-a-day fruits and veggies (and hey there's lots of studies that hint that 7 to 9 a day are even better for you!!!!) that sort of stuff...

but first things first, and for weight loss first is being able to adhere to your deficit, and that will be mostly created in the kitchen... so I would start there!!!!

I weigh monthly and find it to be better aligned with my mental health to do it this way. It requires me to be honest and accurate with my logging, but I don't have to worry about fluctuations as much, especially since I tend to have significant fluctuations with periods (when I say "monthly", I actually weigh about 7 days after my monthly cycle has started, so a "month" can vary. To put it simply, I weigh with the lunar month, lol).

This is what weighing monthly looks like on my chart (I skipped one month, didn't want to deal with it, lol):

At first I weighed myself on a daily basis- I was 260 lbs and had a lot of weight to lose, and weighing myself every day helped me to see patterns- especially regarding water weight and how it plays with your head. It was good to get a sense of what was going on.

Now, however, I’m 175 lbs and I’m weighing myself every other week. I don’t want tiny blips on the scale to affect my mood, and I’m confident enough that if I’m logging correctly I WILL lose weight. As I get closer to goal and the rate of loss slows down, I plan to weigh monthly.

As others have said, it’s a very personal thing.