Crazy question ... maybe first of this kind on here?

Ok, this might sound strange but ...

When I fly over on BA or US Air from Philly into the UK (either Heathrow or Manchester) in October, I plan on taking an urn (actual wooden box with ashes wrapped inside into a large bundle) of ashes and a oil painting that was very expensive and both are way too important to ship or send.  I feel only I can handle them because if they got lost, I would spaz out and probably end up in jail or on the news ... "Crazy American Spaz's" kind of thing.  lol

So, I am wondering where I would find the information to see if customs on both sides allows these things to be transported in carry on luggage.

Any suggestions or insights? 

Most airlines have restrictions on the dimensions of carry-on luggage so it would most likely depend on the size and weight.  Also BA only allows one carry-on...so even if both items are allowed, I'm not sure they'd let you take both.  You're best bet is to call the airline and see what they say (make sure you have the measurements and weight available when you call).  Just keep in mind that most things like that are at the discretion of whoever checks you in...so even if they say it's fine on the phone, that doesn't mean the check-in agent will agree.  If they say it's ok perhaps try to get something in writing (an email or something) from a supervisor ahead of time.

I brought my mother's ashes over in my hand luggage back in 2003. At the airline's request, I had a letter from the funeral home stating what the contents were in case I was asked - which I was. Those x-ray machine operators were really curious (!!), but got all embarrassed when I told them what it was. When I said I had the letter, they said they didn't need to check it.

But this was back in 2003, so I'd phone the airline and double-check with them that it's still OK and to make sure the rules haven't changed.

In the US, I think the ashes have to be in an object that can be scanned (when either packed or carry-on), and it is up to the airline to decide whether it should be checked or not. So call the airline to check, but I am not sure about the UK side.
Sidenote: In lots of places in the US you need a permit to spread ashes.

I transported paintings (Philly>Heathrow) by wrapping it to near-death in bubble tape and then putting it into a suitcase (w/o frame), and checking it (too big to carry on). I have taken small framed etchings, bubble wrapped in carry-ons before. I never saw paintings on the banned carry-on list. As long as it can be scanned, you should be ok. But as you need to call the airline, ask about it then.

I brought my dad's ashes back with me from the states in November 2007. They had to be in a container that could pass through the x-ray equipment. The funeral home had sturdy plastic containers to use for this purpose and they sealed it very securely and provided me with a copy of my dad's death certificate and a notarized letter from the country coroner attesting to what was in the container. This was just in case airport security wanted to open it. I didn't have any problems at any of the airports and when I got to customs in the UK I did declare the container on the customs form just in case. Again, no problems.

and a oil painting that was very expensive and both are way too important to ship or send.

For what it's worth, I shipped several valuable paintings and had no problem at all with it. So if it turns out you can't bring the painting on the plane with you, that's always an option. Just make sure you've insured it appropriately.