I can't help on the shop front (well I could, but postage to the UK might be a nuisance
) but looking at the waterway in the photo it doesn't look that bad to me. I would be tempted to guess that the corrosion is due to water in a cylinder, was the engine left with a plug removed/carbs off ? I'm just wondering about water ingress if the inlet valve was left open and no carbs in place.
I'll second what John has said, skimming TC heads isn't something to do just for the sake of it and if you must do it then take off the absolute minimum.
If this is a Big Valve head then in the UK at least they had a higher compression ratio and a shorter head depth, hence less available metal to remove when skimming. My advice on skimming would be to check to see if it's flat before doing anything because unless the engine has suffered overheating/head gasket failure then it's most probably going to be ok. Even when my water pump failed and caused steam/overheating the head was ok despite the mistreatment.
If you do want to overhaul the head then valve guides are usually the first problem area with oil burning. Valve seats/new valves usually go hand in hand with new guides, that's what I've done with both my engines but it's not a cheap overhaul.
Brian