"International arms trading under certain circumstances is both beneficial and necessary"
A matter of opinion, but indiscriminate trade to terrorist states of the type Britain trades with is not - this is what is under discussion here
Alliances with membership nations by selling them weapons - little more than self-interest - never
"nuclear WMD:"
America has nuclear weapons and it is the only country to have used WMD against a civilian population - ever
With the highly Donald Trump at the helm, the risk of America giving a repeat performance   
There is no argument that America is any more fit to possess nuclear weapons than any other state (wasn't that long ago they were dropping burning petrol on peasants)
You carefully omit Isreal (the country which attempted to make Apartheid South Africa nuclear proficient) from your list - indicates that you believe only extremist states you support are fit to be nuclear facilitated
"the UK at the moment happens to be the sixth largest arms trading nation in the world behind the USA"

Satistics collated by UK Trade and Investment, a government body that promotes British exports abroad, show the UK has sold more arms than Russia, China, or France on average over the last 10 years. Only the United States is a bigger exporter.

Britain is now the second biggest arms dealer in the world, official government figures show – with most of the weapons fuelling deadly conflicts in the Middle East.
Since 2010 Britain has also sold arms to 39 of the 51 countries ranked "not free" on the Freedom House "Freedom in the world" report, and 22 of the 30 countries on the UK Government's own human rights watch list.
A full two-thirds of UK weapons over this period were sold to Middle Eastern countries, where instability has fed into increased risk of terror threats to Britain and across the West.
http://www.independent.co.uk/news/uk/home-news/britain-is-now-the-second-biggest-arms-dealer-in-the-world-a7225351.html
Jim Carroll