I like using Perk to book my business travel as it combines both hotel bookings and rail travel in one place. The hotel booking interface is particularly good allowing me to customise my filters and view the options on a map.
The workflow that happens after I have made my choices - i.e. sending it to my manager for approval is also seamless. Review collected by and hosted on G2.com.
Sometimes the rail travel options is offers aren't as cheap as other providers (eg Trainline) Review collected by and hosted on G2.com.
