Possibly the best thing to do is to call them up and get the fault number so you can keep pushing them on it and keep track of it. imo ,if your paying for 100~160 you should not be paying it if your not getting it. id check about compensation if they are taking their time to send someone out and your going a month or more with a degraded line. (their minimum is 40 at half the price of 160, and your not even getting that)
Bad lines can cause these kind of issues if they were poorly installed or damaged. The fact that you were getting 100 originally on what appears should be a 160 fibre line might mean there’s always been a problem.
Have a look here for some information on what you can do https://www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/advice/broadband-speeds-codes-practice
If the issue is in their network and they can fix it you have the right to exit without penalty, however the rules don’t specify an acceptable resolution time yet (new rules change this but they are not in yet). But that doesn’t stop you from pushing them, asking for a discount or shaming them yourself until they give you what you want (assuming the issue is there’s to fix).
On the ISP side there isnt a lot you can do depending on the ISP.
On your side, Ethernet is better than wifi especially in densely populated areas. You may find changing DNS from your ISP to another provider (8.8.8.8, 1.1.1.1 etc) will be faster, how fast depends on how good or bad the ISP one is.
If you have a lot of devices you might be able to prioritise some depending on time of day etc if you need to within your router.