I was looking around for a GBP exchange rate feed recently and couldn't find anything better than the ECB feed you mentioned.
Equivalent GBP exchange rates are the EUR rates in the feed divided by the GBP rate from the same feed. EUR/EUR is of course 1, and not in the feed, so to get the GBP to EUR rate you divide 1 by the GBP rate from the feed.
I would be most surprised if there were a free source of stock quotes available, at least from the major exchanges. Prices are licensed to aggregators, financial institutions, and proprietary desktop quote software manufacturers.
Exchange rates may be easier to come by-- you found the ECB and the IMF provides a lot of data at [imf.org...] , though that will require multiple calculations.
U.S. exchange rates are available in HTML, XLS, and XML from the Federal Reserve Bank of New York at [ny.frb.org...] . The Bank of Canada provides them in HTML, PDF, and CSV, at [bankofcanada.ca...] . Perhaps the BOE provides a similar, pound-centric resource?