I don't believe we have any right to deliberately obstruct them to prevent that happening.
So if a motorist is doing 25mph in a 30 zone and a queue of cars builds up behind him, do you expect him to pull over and let them pass? (Has this ever happened in the history of motoring?) If not, why would you expect a cyclist to do so? Anyway, it's not a deliberate obstruction, we're just trying to make our way in the safest possible way.
This is precisely what I was instructed to do when I was taking my motorcycle test. In fact, the example given was that even if you're doing 30 in a 30 zone and someone comes up doing 50, you should let them pass when safe to do so, even though their speed is illegal. I think the advice has changed since then, and as to whether anyone put it into practice other than when taking a test, I don't know.
There are signs on the single track lanes, that are A roads in the far north west of Scotland, which say that you should pull in to allow other vehicles to overtake.
I think it is correct to say that everyone is obliged not to hold up any other traffic and at the very least, it's inconsiderate to hinder people unnecessarily.
But yes, cyclists, just like any other road users, have the right to use whichever bit of the road is appropriate. I ride on every part of the left hand side of the road, from on the dotted white centre line, to the left of the solid line right in the gutter. Where I ride all depends on different things. I couldn't tell anyone which bit of the road is best for them to ride on. Every situation is different.
I do try to have a presence on the road (Great big panniers seem to help). I often ride very wide, but move in if someone is coming up to overtake, so most of the time, I'm riding wherever I like on the road. It all depends though. I wouldn't ride in the centre of the left hand lane of the A303 in the rush hour. In fact, I avoid that road in the daytime. But at night, I'd generally ride in the middle of the left hand lane until something comes up to overtake. So even the same road at the same speeds can be very different.
On a very busy, but very wide road, like the A17, I ride well into the gutter. It's a single carriageway road, but is easilyas wide as a dual carriageway. Most of the cars, trucks and motorbikes stay almost in the middle of the lane, which means that they pass me with plenty of room without even having to steer around me.