It's a fairly common phrase in Britain, meaning you are showing an unusually large amount of the well known feature/trait/disposition of a specific person.
It relies heavily on the person being named being known for a specific trait.
It also relies on the person it is being said to/of NOT being known for that trait.
You're coming over all Larry Grayson would suggest you are being unusually camp.
Larry Grayson was a British comedian/TV presenter famous for being camp.
It can also be You've gone all ...
You've gone all Mike Tyson would suggest you are being unusually violent.
You can also use the same idea to refer to yourself
Sorry. I came over all a bit Hitler there (if you'd been shouting for a long time)
ETA: (after comments from Rupe)
I think the source of this phrase comes from another common [BrE?] phrase that people will say when they act out of character which is I don't know what has come over me.
The phrasing in question tells people what appears to have come over them.
In the first example (with a slight rephrasing) You're coming over all Gandhi. If someone who is normally aggressive, acts in a peaceful manner which is 'out of sorts' then one could reasonably expect them to say 'I don't know what's come over me (I'm not normally the peaceful, non-aggressive type)'.
In this "reversal" someone else is telling them what 'has come over them' - In this case it's Ghandi-like non-aggression and that trait has enveloped them completely, hence the need for all, the all applies to you, not to famous person.
You've come over all Ghandi is the same as saying You are usually a violent person but in this instance your normal violent traits have been replaced by those of the famously non-violent person Mahatma Ghandi but I'm sure you'll be back to your old self in a few moments.
I think the Americans have a similar turn of phrase involving your ass. As pointed out by Rupe this is similar but not exactly the same.
I'm going to go all Chuck Norris on your ass