minding my own business at 30mph

I was trundling along at about 30mph in a 30mph zone yesterday (I may have hit 51kph so may have been at 32mph). And the chap following me was hooting his horn at me to get past.
What I haven't mentioned is that I was on my cycle, and it was a car following me.
Of course this would never happen if I'd been in a car doing 30mph in a 30 zone.
Why do car drivers expect cyclists to hug the curb when there are a number of blind driveways, and some extremely well marked potholes awaiting repair (the sort of holes which are about as wide as a cycle tyre - long and narrow) - just to allow them to break the law...?
Of course you can use your horn to warn road users of your presence [Highway code para 92]. But not aggressively.