Interestingly, Elaine, there was a programme on Radio 4 about a year ago about corruption in modern Nigeria. Obviously these things are impossible to pinpoint precisely, but the upshot was that the present system of political corruption, from which much everyday corruption stems, was put in place by the British, who wanted to ensure that their man got 'freely elected' after Nigerian independence was declared. I was gobsmacked listening to that programme (it was on Radio 4, so it must be true! 
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There are other branches to Nigerian corruption which has a homegrown root: the convention for politicians to send public funds 'back home', either into their own pockets or into their home villages and to those who have supported them.
Well, the 'pockets' angle is obviously pure greed, as far as I'm concerned; perhaps in some cases it's greed borne out of a history of deprivation. I don't think that ennobles it: theft and greed is theft and greed, and whatever your sob story, if you are well-off and are stealing from those who have little and who are depending on you to help them out of poverty, that is execrable.
However, the 'home villages' angle stems from a generations-old tradition of having a responsibility towards your community. It takes a village to raise a child, and that child them owed the 'village' a debt of thanks for the sacrifices made for its benefit. It's still theft, but it's got a different slant from what most people assume.
The other side to this is paying one's political supporters - much as here people receive knighthoods or other political favours in return for financial and other support lent to political candidates. So again we come back to yet another point of close corrupt contact between our and Nigeria's political systems! 
I just don't think corruption is something that can be measured. By necessity it is usually so well disguised, so underhand: where would you begin if you wanted to quantify or qualify it all? So it really is wishful thinking to say with certainty that one country is more or less corrupt than another.
A Zimbabwean friend once commented that the thing that makes it look as though some countries are more corrupt is that their systems have fewer controls than those who appear to be squeaky(-ish) clean. Politicians will always push the system as far as it will allow them. Nigerians are gregarious people so it follows that whatever they do, they do it in a grand style. So corruption there is far ballsier in nature.
And what of the UK? I heard a story some years back that EDF, the French energy company, was failing badly in France at the time when they won the major contract here which they still own to this day. At the time, Andrew Brown, a brother of the then UK Prime Minister, the Honorable Gordon Brown (I use that adjective ironically in this context), was the head of PR for...EDF. Perhaps the whole thing was squeaky clean. Perhaps the Browns refused to talk shop or give each other a helping hand. But you have to ask these questions, don't you?
I am sure you can come up with your own list of UK public scandals over the years, you don't need me for that. Let alone the people who get jobs because they sleep/ drink/ snort coke with the right person; the talented people who get pushed out by insecure managers, and all the other everyday normal and ignored instances of corruption. We can talk about this stuff till the cows come home. We just can't measure it. And we should never, ever be complacent about it.