To me there is an advantage to being in lane one.
You only have traffic on one side and unless the traffic is very heaving joining traffic isn't difficult to deal with.
There is an even bigger advantage to being a centre lane hog - no-one else exists!
If you read the introduction to the Highway Code, it states that the words "Must/Must Not" are used to state that a rule is law and other words like do/do not, should /should not are only advisory.
My interpretation is that undertaking is legal (providing you stay within speed limits).You will not be prosecuted for a breach of the Highway Code, but, if an accident happens because you don't follow a "do/do not", any subsequent charges will be more difficult to defend.
For the record...a dual carriage way is not a two lane road (in same direction), it is a highway with physical segregation between the opposing direction lanes.ie two single lanes with physical segregation between the lanes is a dual carriageway (and thus a 70mph limit).....I learnt that on a speeding awareness course, lol.
And did you know the speed limit on a derestricted dual carriageway was 70mph, the amount of people who swear it's 60mph is astounding.