My wife is Guyanese and she tells me that in Guyana they are taught to pronounce "to" as an American would pronounce "toe." Guyana was a British colony (the most recent invaders) and their educational system and language are mostly British. Is this the way the word is pronounced in British English?
In Southern Standard British English this word has two weak forms and one strong form. This applies both to the preposition to and the to that we see before infinitives.
The strong form is /tu:/ (like the word too).
The weak form is /tə/ before a consonant (like the last syllable of pasta) and /tu/ when it occurs before a vowel. Unlike stressed to, this last word has a short vowel. The vowel quality may be like the GOOSE vowel or like the FOOT vowel, depending on the speaker.
The strong form is used when the word is stressed, or when it occurs without its following noun or verb.
So we see the strong form therefore in the following sentences:
- I want to.
- Where are you going to?
In the first sentence the rest of the implied verb phrase after to is missing. In the second sentence the Complement of to is represented by where which has been moved to the front of the sentence.
In the following sentences we will most likely see the weak form /tə/:
- I asked to go.
- I've been to the shops.
In the sentences above we see to occurring with its following verb and noun respectively. Both occurrences are before a consonant.
In the following we would expect to see /tu/, where the word occurs before a vowel sound:
- I need to ask.
- I've been to Amsterdam.
Some speakers of very modern RP always use /tə/ for the weak form. Where it occurs before a vowel these speakers usually use a glottal stop at the start of the following word. This is called a hard attack.
Note that there are many other varieties of British English apart from Southern Standard British English. These may have different realisations of this word.