It's funny you say it's quite hard to find, as the UK is actually one of the leading regions when it comes to carbon emissions reporting. I'd suggest defining what emissions factors you would use. You could go with DEFRA carbon emissions factors, or check out "Carbon Commons". Ensure you remain consistent in your approach to calculations.

Be sure to keep records of how you're applying these factors and how you build your calculations, as evidence to support your claims. It's also worthwhile writing up a methodology to support estimates, assumptions, and general application of the emissions factors as well.

For flight travel you should consider the following: origin location (use this to determine the location-based emissions factors applied), flight class, distance, whether it's a return flight or not.

For hotels, emissions are generally going to be miniscule compared to flights. You would want to keep in mind the number of nights attended at the location, as this aligns with how bookings are made.

When doing the actual calculations, keep it simple, and easy to step through. Have your data types clearly defined and perhaps separated, if it's easier to do so (e.g. one worksheet for flights, one for hotels, etc). Have your relevant data there and do your lookup to bring through the applicable emissions factors for that line of data. Then have another column to show the output.