The two terms are International English for British English (and we are the franchise holders) and Universal English.
As a UK born person, I would question the need for an International English version as well. All the big companies, Microsoft, Adobe and suchlike, seem to produce one version and we have learned to accept the misspellings.
What I would do is get your help reviewed by someone raised in the UK or who is very familiar with the different words used. I found in the US "spanner" and "lorry" are not commonly known. The US terms of "wrench" and "truck" are less common here but would be understood. "Faucet" on the other hand would probably get a blank look here. I would take a look at the terms and consider whether or not you really do need two versions.
If you do need two versions, then the simplest solution would be to use variables where the terms differ and generate two outputs. Another language would require to copies of the project bringing in the attendant issues of maintenance going forward. Using variables would be much easier.
See www.grainge.org for RoboHelp and Authoring information
Thanks Peter. Yes, I fully agree but unfortunately it's a customer request that we have to adhere to. I was considering variables or conditional tags as possibilities to keep it all in one project, but I agree that variables are probably the best option. I've decided to go with separate projects for the French, Spanish, German and Portuguese languages unless anyone can give me a particular reason not to.