''To'' and ''two'' and also ''For'' and ''four''.

?!
As an American, a Brit saying "tune" sounds just like tyoon to me. Not choon. I hear the _exact_ same initial sounds in almost every case. (Th being the exception. Th becomes just T in some UK dialects.) Is this something not in RP? Have a sample I could listen to?

I've been listening to the BBC on shortwave for the past twenty minutes now trying to understand what you mean, but I just can't hear your pronunciations...

Let me try to make it clearer.
Many words with an [u:] sound spelled u, ew, or eu after a [t, d, n] sound have an inserted [j] sound before the [u:] in standard British English. So "tune" sounds like "tyoon", and "new" sounds like "nyoo". Pronouncing "choon" for "tune" is somewhat of a London thing, and is also popular among young English speakers in southeastern England.

Do they also pronounce ''two'' as ''tyoo'' or ''choo''?

<<As an American, a Brit saying "tune" sounds just like tyoon to me.>>

As a Scot Brit my version is "['tun] with the vowel almost identical to the French one in "du" or "mur"...short and sharp.

On TV yesterday I saw an American politician salute and he said what sounded to me to be: "I am John Kerry and I am repording for doody" ;-)