I'm a tad bemused regarding this thread's argument over 'mandate'.
Firstly, the UK does have a constitution. The vast majority of it is written i.e. Magna Carta, Act of Union 1707, Parliament Act 1911 etc. So it is wrong to say that the UK has an 'unwritten constitution'. However, some of the constitution is unwritten i.e. the constitutional conventions.
It would be correct to say that Britain has an uncodified constitution.
Secondly, the vast majority of the power of 'The Crown' rests not with the Sovereign but with Parliament.
In part due to this, it is a long established convention (i.e. part of the constitution) that on the advice of the (outgoing) Prime Minister, the Monarch invites a member of Parliament who is most likely to command the confidence of the House of Commons.
In 2010, on the advice of Gordon Brown, the Queen invited David Cameron to form a Government. He had the confidence of the Commons, therefore succeeded and became Prime Minister. He had a constitutional right to be Prime Minister. As Prime Minister, he advises the Queen on who to appoint and dismiss as ministers. Therefore, after 2010, HM Government, in which David Cameron was Prime Minister, had a constitutional right to govern.
After the 2015 election, Mr Cameron remained as Prime Minister and First Lord of the Treasury as he continued to have the confidence of the Commons. As stated above, as Prime Minister, Mr Cameron advises the Queen on who to appoint/dismiss as ministers. Therefore, HM's current Government, in which David Cameron is Prime Minister, has a constitutional right to govern.
You might not like this constitutional right to govern, you may think it is flawed and requires change, but the current Government does have a mandate to govern.
Since then the Thatcher government agreed to the Single Market on our behalf and the Major Government agreed to aspects of the Maastricht treaty, giving the UK an option not to join the ERM/Euro as it became.
It's an aspect of the Single Market agreement that the Tories are banging on about in terms of welfare rights. How strange that Thatcher didn't see that one comiong, or didn't see it as significant in the greater scheme of things.
Ironically, with the Single European Act in 1986, Mrs Thatcher presided over the greatest transfer of powers from the UK to Brussels. Far greater than Maastricht.
Mrs Thatcher was always an advocate of the a fully integrated Single Market - she (and many others on the continent) probably didn't realise that this meant Brussels would become far more powerful.
I believe in the noughties, she stated that she regretted the Single European Act.
And as far as I recall UK governments of all colours have pushed for enlargement of the EU to take in some much poorer countries. This is the main cause of the immigration which is now a concern to a significant proportion of the public.
Quite right, again this policy started in Mrs Thatcher's time in office. She (and subsequent administrations) encouraged enlargement of the EU to the ex-communist countries as she felt that they would be a brake on European integration.
As the populace of these countries had experience in being ruled by a foreign power, she felt that they would fight to keep their newly found freedoms and powers within their sovereign nations and not hand them over to Brussels.
She was in part correct. Aside from the 'old' eurosceptic countries of the UK and Denmark, most of the eurosceptism in the EU is found east of Germany*. However, this has not stopped the power of the European Commission from increasing.
*It has changed recently due to the Euro crisis with most member states nowadays having strong eurosceptic movements.