Should it be "was" instead?

Depends on where you live. The UK say one way and North America says another, depending on whether they see the unit (police force, PTA, etc) as a single or plural entity. I don't think it really matters; unless you're doing an English Grammar exam, no one's going to be fussed about it.

No

"Singapore Police Force (SPF)" is plural, and so "were" is correct.

If you changed "Singapore Police Force (SPF)" to "Singapore Policeman" then "was" would be correct, because "Singapore Policeman" is singular.

Yes - if you are British. No - if you are American.

Thank you for setting the record straight, I sometimes forget I'm on the world stage here

Singapore, and the OP, uses standard, not American English.