I know it's a bit late, but since no-one has actually answered the second question... any DECIMAL stamp is still valid at face value (of course, this means QEII only). ½p stamps still hold their value, but the total amount will be rounded down if it isn't a whole value. eg a 3½p stamp on it's own is only worth 3p, but two of them are worth 7p. I do a fair few sales on ebay, and have a few hundred sets of the 1979 centenary of the death of sir rowland hill mini sheets, four stamps with a total value of 49½p - so all they need is an extra 1p stamp to make 2nd class, or stamps to 10½p for first.
Regarding NVM stamps (none value, 1st, 1st Large, 2nd, E, Worldwide etc) these are equivalent in value to the current rate they represent - an E stamp, even though not produced for a few years, is worth 88p at the moment and can be used towards any postage costs not just letters to Europe. A 1st of whatever era is worth 60p, and can have an extra 9p added to it to make a 2nd large, or 30p added to make a 1st large.
Going to pre-decimal stamps, the whole pound value stamps are still valid for QEII and KGVI, but shillings and pence are not.