With regard to the original question, I would say that in a number of cases - principally houses and the stock market - the wealth never really existed.

Take housing. Imagine starting with one million houses, each with average value £100K. Valuation of total housing stock £100 billion. A few (doesn't take many) houses change hands at - say - £110K. Now the valuation of total housing stock is set at £110K x 1million, i.e. £110 billion. Hence, we see an apparent increas in the country's wealth of £10billiion, despite the fact only a fraction of the house changed hands at the higher price and there is NO more money in the country than at the start point.

With houses, when valuations increase, people feel wealthier, borrow against this apparent increase in their wealth, spend more, business do better, share prices go up....everything feels great.

Same occurs with the stock market. Just a few of a company's shares need to trade at a higher price and the company (due to having many million shares) is valued massively higher.

I don't have an answer to all this, but it suggests (to me at least) our method of valuing "wealth" is wrong

The answer to the question of "where has the money gone ", is perfectly simple. The missing hundreds of billions of pounds were spent by Tony Blair aided and abetted by his reckless and feckless cabinet, all of whom came to power after being in the political wilderness (where they now reside - hopefully for eternity) for 18 years. So enormous was Blair's ego that he willing fell under the spell of a foreign leader in exchange for the adulation he received from the politicians on America's Capitol Hill. The consequences of which included joining the United States in a war that history had already told us could never be won and another war which under UN Resolution 1441 was illegal and predicated on a fabric of lies told to parliament. These two wars have cost this country not only the relentless loss of of hundreds of the finest men and women soldiers in the world but also a conservative £20bn pounds to date.

A further £15-20 bilion pounds have been spent by Gordon Brown on two non nuclear powered aircaraft carriers (flying American VTOL aircraft), which when built will see one immediately mothballed and the other hopefully fully operational by 2025.

Further billions were spent on benefits for half a million illegal/legal immigrants Labour deliberately swamped the country with .

More billions of pounds were squandered by Blair and Brown on paying benefits to people who existed in a twilight world where successive generations of families never worked and amongst whom many unmarried men and women lived off the benefits gained from breeding like rats .

Meanwhile PM (ex Chancellor of the Exchequer) Gordon Brown was quite happy to ignore the financial if not criminal actvities perpetrated by city bankers as long as they poured in the taxes to fund Labour's lunatic policies, right up until the balloon finally burst in 2008. All which has brought this recently great country to its knees.

Me too, though the majority of the electorate chose Blair/Brown's idiotic lack of control over spending and use of all our "rainy day" money. It is no wonder we are so impoverished. Every day there was a new announcement about how much was being spent on this or that, as if the spending was a better measurement of their success than what was achieved.
It would be interesting to see the numbers relating to foreign aid given by those countries (worldwide) where greater wealth has been created in recent years. (I am not making any assumptions, here.) If we can help eliminate real poverty then we should but it seems that as a nation we are borrowing to give at a time when we are struggling to avoid sinking ourselves.
I heard that the French were pleased with their increased exports of wine, to China.

That reminds me, with the passing of Jimmy Saville, as well as Bob Hope, Johnny Cash and Steve Jobs, they say:
"No Jobs, no Cash, no Hope and no longer anyone to fix it for me." (Apologies to those who have already heard it. This is not an original.)