I'm surprised there was a suggestion that P Type should be sprayed to get the correct film thickness. The data sheets say "P Type 1K Topcoat is a single component brushing topcoat" and under "Application Process" it says "By brush appy 1 coat" which is pretty clear to me.
I have to say that when I emailed for advice, stating I was an amateur NB painter, I got a rapid and very helpful reply. Of course, writing for help when there is a problem, particularly if the problem is with the product, is different. It is also a chance for the company to demonstrate that it really is interested (or in your case isn't) in its customers and reputation.
BTW just for completeness, the data sheet says Rapid Excel primer comes in buff, off white, light grey and dark grey. I'ff not sure what colour buff is or what it would go under and off white and various shades of grey don't get you much further than black and white and mixtures thereof but I thought I'd mention it.
Finally, now they don't do E500 synthetic undercoat, which was also designed to be brushed, they sell only primers designed for spraying. Perhaps P Type will be the next to go and they will concerntrate on spray coatings only. Perhaps they would let us all know so we could begin moving to another brand if that is their intention.