I have used the Douglas and Ribble many times, sailing as well as in motor cruisers, and I once had to take a Dawncraft 25 over with only 8hp. (underpowered, max about 4.5mph)
I came out of Tarleton the day before on the high tide (slack water) and had arranged to stay over night on the pontoon at Hesketh Bank. The stay was pretty cheap for one night and it's an interesting mooring with water on the end of the pontoon and toilets, showers, etc ashore. I actually left the mooring two days later due to bad weather on the middle day (safe on the pontoon though), as soon as I was afloat. I stayed right to the side of the river out of the way of the fast flowing centre of the river (the depth sounder was useful there) and creeped down the Douglas at a very low speed, but still reached the 5 mile perch (pictured above as the Astland Lamp) well before high tide, in fact they were only just letting them out at Preston, so once around the perch I was WITH the tide most of the way to the entrance to Savick Brook (the canalled bit that leads from the Ribble to The Lancaster) and made very good time.
I am not suggesting a narrow boat stays as far to the side as I was (remember I had the depth sounder), and some sandy bit jut out but I was on a rising tide, but by breaking the journey I was able to take a very slow boat across without the initial hard punch that is there leaving Tarleton Lock. If one were to leave Hesketh Bank about 15-20 minutes prior to the time you would normally arrive there, you would be starting on a cold engine and have no need for the initial 'thrash'.
I don't suppose this is any substitute for a well cooled engine though.
I spent many years 'doing' the Link in a Norman 20 with a 20hp engine which would do about 12mph and it was never used at flat out as it simply wasn't necessary.
I think the answer is to know your boat and it's limitations.
Also take Harry's advice at Tarleton, he is THE expert on those rivers and what he says is ALWAYS to be listened to. No one knows them better.