Is it illegal? x2

Interestingly enough the British Horse Society recommend that riders ride two abreast so that drivers do not attempt to overtake them when the road is too narrow.

Not much fun when you get behind a pair of four legged shit monsters on a country lane on your bike though.

Usually when I cycle with friends we ride two abreast but then if someone can't get past we just go into one line but far enough apart to annoy them

Short answer is that Suffolk drivers need educating.

Riding 2 abreast annoys some drivers because they feel you shouldn't be "in their way". Fact is you're a road user just as they are but of course an obstacle 2 cyclists are much more difficult for idiots to push past into oncoming traffic.

There's no need to be inconsiderate but your safety ranks way above some impatient jerk's need to join the queue up the road 5 or 10 seconds earlier. In a group 2 abreast creates a shorter bunch of riders than single file so actually makes overtaking easier, but trying to make some pig-headed drivers understand this is impossible so I suggest you don't bother trying.

@londonplayer, yes Primary position AKA 'taking the lane' is a key element of defensive riding, particlarly in urban traffic. It was outlined long ago in John Franklin's book Cyclecraft. Bikeradar has an article on road positioning and other training orgs will have similarly helpful advice online.

Using it takes a bit of nerve but it prevents you being cut up e.g. on the approach to pinch points like refuges, pedestrian crossings, mini RABs and so on. And never ride in the gutter, you're giving everyone behind you the impression that you are scared and are giving them just enough to squeeze past. I generally ride in the left hand car tyre tracks - you are much less likely to encounter stones, glass and other detritus as well as avoiding drains. You are also more visible to those waiting at side roads.

Am I right in thinking that you are allowed to ride in the middle of the lane if you feel like it? Sometimes it's definitely safer to in London.

It's not illegal. Rule 66

https://www.gov.uk/rules-for-cyclists-59-to-82/overview-59-to-71

Not illegal - Highway code says this:
66

You should

-keep both hands on the handlebars except when signalling or changing gear
-keep both feet on the pedals
-never ride more than two abreast, and ride in single file on narrow or busy roads and when riding round bends
-not ride close behind another vehicle
-not carry anything which will affect your balance or may get tangled up with your wheels or chain
-be considerate of other road users, particularly blind and partially sighted pedestrians. Let them know you are there when necessary, for example, by ringing your bell if you have one. It is recommended that a bell be fitted.