Please use mirrors

An offence? I don’t think so. Have you a link we can all learn from?

Of course, it’s possible the vehicle had a mechanical problem restricting it to 'limp mode'.

Highway Code Rule 169

Do not hold up a long queue of traffic, especially if you are driving a large or slow-moving vehicle. Check your mirrors frequently, and if necessary, pull in where it is safe and let traffic pass.

I appreciate it can be very frustrating following a slow vehicle over a long distance but you mentioned it was 20 miles where the difference between driving at 40 mph and 60 mph is about 10 minutes. I suspect that many motorhomes even if it is legal to travel at 60 mph the majority will be travelling more slowly, although I would agree that 40 mph is a bit too slow. It did seem you didn't really leave yourself enough time, especially for something as important as hospital appointment. There could have been any number of reasons that you could be held up?

David

As I underland it the not all the rules in the Highway Code are law, only those rules which use the word must or must not.

Many of the rules in the Code are legal requirements, and if you disobey these rules you are committing a criminal offence. You may be fined, given penalty points on your licence or be disqualified from driving. In the most serious cases you may be sent to prison. Such rules are identified by the use of the words ‘MUST/MUST NOT’.

https://www.gov.uk/guidance/the-highway-code/introduction

So in my view (and my legal expert) it's not an offense.

Also there are rules relating to overtaking, such as safe and legal to do so and as the OP states and due to holiday traffic nobody could overtake which seems to imply there as a constant stream of traffic coming the other way anyway?

While driving too slowly may be classed as careless driving again the OP states 40 mph in holiday traffic so I would think that doesn't come near.

Should the MH have pulled in? Maybe, but perhaps 40pmh was the safe speed for the conditions, or perhaps they too were in a hurry just like the OP, and not all MHs are on holiday are they? Perhaps 40mph kept a safe minimum distance from any vehicle in front and no point therefore in pulling in?

But not an offense I would say.

If he was travelling at 20mph, I would say the Rule applied as it does to large and slow moving agricultural machinery but 40mph is hardly in that category. Rule 169 is not a 'must/must not' rule as far as I can see so is not law.

David makes a valid point above.

And if the motorhome concerned was over 3050 Kg unladen it would legally be restricted to 50 mph on a single carriageway..... iirc

The joys of BH traffic eh!

It might be classed as driving without reasonable consideration for other road users but I doubt it. As has already been said - 20 mph and that would be sustainable but not 40 mph.

Personally I would sacrifice fuel consumption in such a situation and keep up with other traffic especially in rush hour traffic on a BH weekend but better still I would not be on the road!

Chris 

There can also be the problem of pulling out again in busy traffic. I remember whilst towing pulling over as we were holding up traffic by traveling 45 / 50 on a 60 (for solo cars) road, and then not being to pull out again safely. Nobody wants to let a caravan out in front. 

And taking into account the condition of todays pothole roads and motorhome suspension even 40m.p.h. might be pushing it.

peedee

I could be classed as  'inconsiderate driving'. This is defined as driving a vehicle on a road or in a public place without reasonable consideration for other people.  That is an offence but I think that what has been been described by the OP is very unlikely to be prosecuted.  Frankly I suspect that as road user we all see far worse on a daily basis.