^I agree. Polyethylene Glycol 3350 is the active ingredient in the product Miralax. I've tried several laxatives over the years including pills, liquids and suppositories. I've found that the most comfortable and effective means was the Miralax. I'm not sure what the brand will be over in the UK, but I assume Boots will have something similar. So, that's the first choice, but there are other effective methods as well.
My second choice is almost as effective as Miralax and maybe just as good, but I have less experience with it: Magnesium Citrate. This is also a drink. Drinks have an advantage over pills for reasons that you can probably imagine; they reach the problem area faster and more easily. Magnesium Citrate worked fine for me, I was cleared out by the following day, but went that day as well.
I really believe that one of those two will work for you. I will bet money on it, but if not, there are other options. Senna is an all-natural laxative that works pretty well. The next step would be something called Bisacodyl. It will cause you to have a pretty powerful shit, which can be kind of uncomfortable, which is why I choose not to use it. This is probably too much information, but I might as well just say it. Once, I took Bisacodyl, as I hadn't shit in about 6 days. Was heavily dependent at the time. I ended up shitting in my sleep around 8 hours after taking the pills. My stomach was in knots and quite painful. Luckily I didn't shit the bet, but boy was I close.
These are your options Fermonos. I wish you luck!