Lexy - the questions you have asked are about what an academic would call 'political economy' and really is what the job of politicians is all about or should be about. Balancing the economic, social and political issues for the benefit of all.
An economist purely looking at the economics would say that the UK economy as a whole will benefit from cheap labour flooding in as it will become more competitive and hence bringing higher economic growth. More especially if we focus on the things that we have a comparative advantage in already like banking and finance, legal services, high tech manufacturing (eg Formula 1 racing cars around Oxfordshire) , luxury goods (eg whisky, fashion) certain aspects of film making and production, education, pharmaceutical research, oil & gas, mining, construction.
Problem is that not every person in the economy benefits from the type of economic growth we have experienced in the last 10 - 15 years. Only an elite group in th eprivate sector really benefit along with higher grade civil servants who are paid out of the taxes that these industries produce.
A higher NMW would benefit those on lower grades in the public sector and would also benefit those that can keep their job in the private sector. Some people argue that it might though destroy other jobs that private employers cannot afford to pay above current NMW levels. The danger is that we destroy jobs with a higher NMW. Higher NMW would also draw in even more immigrants.
I do think we need to close the UK borders to anyone who is not in the highest skill and highest paid jobs (ie like Aus/NZ and Canada).
I dont like pay caps and wage controls but I do think that freely allowing skilled people to come in to compete down the pay of those in top jobs (which are frankly out of control) while keeping out those who would drive down pay of people at the bottom would be a good thing.
If we could close borders to low skill or semi skilled immigrant workers but kept it open for high skill workers then we might well have a case for raising NMW and indeed wages might be bid up at the bottom anyway with less competition from immigrants. This would help close the gap between top and bottom of the pay scale - which is in real danger of violently rupturing society if we dont do something.
It is the job of politicians to balance the politics and economics of this. The incumbent political parties are not doing that but UKIP have a message that is increasingly people like and want to hear - especially among what was the old 'working class'.
Labour are just as frightened of UKIP as the Coalition parties and they should be.