EPC nightmare

don't listen to Halfords

Obviously on a forum we can only guess as we do not know the car, its mileage and how it has been treated/serviced in the past. I also assume that the problem appeared before the most recent engine oil change. I’m assuming your oil is 5W-30 as put in many modern cars.

Do you think it is possible that the engine has got a little “gummed up” in the past with infrequent oil changes? This might restrict flow in some areas or restrict the oil pick-up. If you suspect this I would conduct another oil change using one of the pre-drain flushing additives and also put in another filter. On the topic of the filter (I’m not familiar with your particular one) but usually there is a valve (and sometimes rubber o-rings on a filter stalk depending on the type of filter) whose job it is to maintain pressure in the upper engine area by stopping engine oil just falling to the sump. A valve fault or worn o-ring (if fitted) can cause an issue.

You have a new oil pump and oil pressure switch so I’ve discounted these and assumed that you do not have excessive bottom-end engine wear. Best of luck.

If there's anything blocking or restricting the oil pick up no amount of flushes will fix it, there's only one way to deal with it and that's taking the sump off and cleaning or replacing the pick up pipe. I've dealt with this on two occasions on cars that had this problem, it's good practice to clean/replace the pcv system at the same time.
Only a genuine oil pressure switch should be used, also check the wiring for the oil pressure switch.