I live in a fairly rural area and at this time of year I see plenty of farm vehicles on the road travelling from field to field, they are working, so I am prepared to be patient when stuck in a queue.
However, the Highway Code does state that when a long queue builds up the vehicle should pull over when safe to do so.
I followed a tractor for around 6 miles up quite a steep hill, double white lines all the way up. There were probably 7 or 8 cars in front of me and even more behind. The tractor drive made no attempt to pull over despite there being several opportunities to do so, including a short parking lay-by.