I think you'll find that entirely depends on the type of bread. OP specified Warburton's white [an unhealthy choice in itself] but there are other loaves which will give you upwards of a good 110 cals per slice. Bet let's go with 100 cals for simplicity's sake.
Now, lets say that OP has four slices of bread at 100 cals each with a tin of Branston beans which is 350 cals per can. Would average at around 750 cals just for those. Let's say they add butter for around another 35 cals or so. This would mean this one meal is nearly 800 calories.
Now, if we multiply this by three the OP is going to be eating around 2400 cals a day. Depending on his lifestyle and his build, he would either gain weight or stabilise on this diet. It's unlikely he'd lose weight unless he was very active. More than likely he would gain weight.
So, without any information on the OP let's say he's around 5,10" tall and with a weight of 12 stone and a sedentary lifestyle. A lot of assumptions, clearly, but it's important to illustrate the point.
His BMR depends on his age. Hard to guess. Let's say 40 for an average.
This means that the OP would, at rest for the day, burn around 1729 calories at rest, though more likely something in the region of about 2075. This gives an excess of around 325 calories unused which will cede weight gain over time.
These figures can be represented in formula form or for convenience in a QR code, as below:
The figures above are only examples. But they do go to show how having four slices of bread can lead to weight gain if the OP has them for three meals.
What if OP reduces to 2 slices? This would mean a reduction of calories even with the same amount of butter by 600 cals giving a massive reduction to 1800 cals a day.
Given the OP would maybe be burning around 2075 cals on an average day then yes, this would lead to weight loss, although gradual.
Of course, no-one is going to eat beans on toast for three meals a day, though someone out there might.
