They are all very good schools. I am currently studying at RGU, although I'm not on any of the courses you listed. I don't know much about London South Bank University, but I know that Heriot Watt is rated very highly.
From what I have heard Univeristy of Salford is one of those schools that created their Petroleum and Gas programme out of desperation, just to attract tuition fees income.
You should also bear in mind that London is a mad place to live in, what with the high rent prices and transportation costs.
Personally, I'd advise that you extend your search to other schools (or better still, other countries). The USA and Canada have schools offering excellent oil and gas programmes. You may want to check out Texas A & M, Colorado School of Mines, University of Oklahoma, Phoenix and the rest of them. www.findaschool.org is also a wonderful website.
And finally, from experience, I know that studying in a foreign country (especially the UK) is a huge endeavour. Please make sure you consider every factor, especially your finances before reaching a final decision.
God help you through your decision-making.