Intentionally made as the internal wiring often causes the speeds to plummet if the internal wiring is poor quality, runs by electronic wiring, goes past electrical appliances, has bell wire connected etc etc.
If you had a full engineer install, they will usually relocate the master socket if the location makes no sense. I am surprised they have left you with a poor setup.
You say you already have an MK3 filter - then you later say you are thinking of buying one of these https://www.run-it-direct.co.uk/btvdslfaceplate.html
Do you realise the link you are thinking of buying is also an MK3 filter, which you state you already have?
I suspect you are just removing the very front faceplate and just not seeing the data extension terminals.