Hello Flashman, welcome!
For a catalogue with new up-to-date prices you might need a new Stanley Gibbons. SG lists more varieties and goes into greater detail for British and Commonwealth issues than any other catalogue, I hear.
Generally speaking Scott catalogue or another general worldwide catalogue may be OK. Depending on whether you want to get into the detailed varieties (if any).
Pricing is another question. SG's prices are considered to be somewhat high and Scott are more geared to the US market.
For buying online consider
ebay (click worldwide) or Delcampe for worldwide sellers or Bidstart or others here in North America.
ebay may be a source for older used catalogues, if you don't need to be exactly up to date with everything, and for 70 year old stamps, not much changes really (although there are always exceptions).
Most KGVI stamps are relatively inexpensive as stamp collecting was in a good period in those years and lots of stamps were printed to meet (and exceed) the demand.
Condition plays a big part in stamp valuation. Well centered, good color, nice gum (if mint) or nice cancel (to your own preference) (for used), no flaws or faults will make for a more valuable and sought after stamp.
Remember to use the search button yop right to search this forum for KGVI stamps. Not of other sites otherwise.