So,
Could I apply for tax exempt status in the UK and US if I were to revive the old Greek traditions?
It depends. The tax exempt status only applies to religious activities done in the name of an organized entity. I use the term organized lightly, think along the lines of registering a coproration. However a number of people have run afoul of tax fraud for claiming their organization was a religion when it was just a tax shelter. That guy who flew a plane into an Austin, TX building earlier this year got in trouble for that sort of tax fraud. I could see Greek Neo-Pagain or Greek Pagan Reconstructionism organizations already existing in the US that receive tax exempt status.
That Homeric stuff always appealed more to me than Christianity.
I have always enjoyed the mythology of both.
Do all religions get tax exempt status in the US?
I cannot think of off hand which organizations have been denied in recent decades that most would could consider an actual religion. Scientology gets investigated fairly steadily, but mostly for specific practices that smell of tax fraud. They were denied for something like 25 years in the beginning.
Does that include Satanism?
The Church of Satan (The LeVay one) disagrees with the notion of any religions being tax exempt and has not tried to apply, as far as I know.
Are the Freemasons legally a religion?
Freemasons demand members be theists, but beyond that do not make religious claims. I doubt that the main Freemasonry or related/offshoot organizations have attempted.
I'd been pondering these questions, sorry to fire them off all at once.
Keep in mind that income earned as an official of a church or organization is still taxed. What the exemption generally applies to is revenue gained (donations, charges for certain services), property tax, sales tax (I think this depends on the specific sales tax in question), and others I likely missed. Tax exempt religiouns are also not allowed to have profits or shares with the tax exampt portion of their funds. In other words, an individual cannot be an investor in a church and collect dividends and such. Businesses done in the name of religious services take are where the tricky bits come in I think. I have not looked into it that much. For instance a book store as a legitimate business would not normally qualify for tax exempt status on its receipts but an adoption service most likely would. Basically the types of services that could qualify as a tax exempt charitable institution regardless of a religious affiliation.
Btw, here is a link explaining a bit more about church tax exempt status:
http://www.irs.gov/charities/churches/index.html
More specific information:
http://www.irs.gov/pub/irs-pdf/p1828.pdf
As I have no professional experience or training in any of this, I very well could have gotten something grossly wrong.