Can you see the difference? Firstly Brown's lectern is dark oak and Cameron's is light ash. I am not sure what that means design-wise, probably nothing. Secondly, and I think more importantly, Brown's lectern (if it had any adornment) had the URL of Number 10. Cameron's has a crest. In other images it is clear that the crest that Cameron uses is the royal coat of arms.
(In my opinion, the URL looks more modern, the crest is too detailed with the effect that you cannot quite see what it is: from a distance it is just a blob.)
The items on a coat of arms have a lot of significance and in particular, coats of arms usually have a helmet of some kind that indicate the status of the family. The crest Cameron uses clearly shows that it has a crown as the crest, furthermore, the supporters (the lion and unicorn) have crowns. This is clearly the Queen's coat of arms, and the Queen (or institutions that represent her, like courts) is the only person who is allowed to use it. So why is Cameron using it?
The Queen is the head of state as the constitutional monarch, but the executive (and Cameron is the head of the executive) is separate. Indeed, the country fought a civil war to assert that Parliament and the monarch were separate. Parliament uses the crowned portcullis as its emblem (as used on the old style 1p piece before the Royal Mint gave us the horrible jigsaw puzzle piece that newer coins show) - and the crown is used for the House of Lords. Should Cameron, as Prime Minister, use the Parliamentary coat of arms? Perhaps not, but constitutionally he is closer to Parliament than the monarch.
I cannot find a single case of the previous government using the royal coat of arms as the symbol for the UK government, yet I find the coat of arms everywhere on the current government's websites. This logo is used throughout:
A little more searching gives me a civil service web page which explains the re-branding:
"The identity embodies integrity, trust, impartiality, quality of service, and professionalism – values thatare an integral part of the civil and public service. Its use should be governed by similiar values and be honest, effective, impartial and accountable."
I am not sure that Cameron (a politician who owes his status to the fact that he is the leader of a political party) can be described as impartial. This reinforces my question of why he uses the royal crest. The branding documents (irritatingly, there are several, each with a handful of pages, why couldn't they have been put together as one document?) further say:
"the Royal Coat of Arms can only be used by organisations that have Crown immunity from prosecution"Does this mean that Cameron is immune from prosecution? I certainly hope not!
I think the reason Cameron uses the royal crest is quite simple: it is an indication of his inflated opinion of his own importance. He has royal blood in him, so he he thinks he is royalty.