Why not busses? If you sail a carrier packed with aircraft to a foreign land you're really boxing yourself in as far as tactical options go. There's really only so much you can do with military attack aircraft and no matter what Admiral Blueblood says, none of those things are going to be novel, or even creative.
But sail an aircraft carrier full of big London busses to a foreign land and nobody will know what to do. Are the busses a gift? Have the British woefully misjudged the available surface road infrastructure in Kerplackistan? Have the British discovered anti-gravity technology and no longer need their aircraft to be aerodynamically efficient? Are the busses full of migrants? Are the busses Transformers?
See, nearly limitless options, none which justify attacking the bus carrier preemptively. Only a fool would launch an attack on such an enormous what-the-fuck. You can't shoot at things unless you know what's going on and nobody is going to know what the hell is happening when HMS Mass Transit anchors offshore. For a few days, even longer if you refuse to communicate, the British Navy will once again control the Seas.