London #148 bus - what is the estimated time it would take to go from Lancaster Gate to Westminister Abby, at 7:30 am on a mid-July Saturday morning?
TfL - Transport for London, who run all the transport of all types in greater London, have a fabulous Journey Planner. It may not be perfect, but it isn't far off. https://tfl.gov.uk/plan-a-journey/
Pop in your origination point, your destination, date and time (remember the default will be London time at the time you look) and you will get a very good selection of options.
You can modify types of transport, walking time, all sorts.
Default answer - 12 minutes
Another great app to use for London Transit is CityMapper which will give you real time information as you travel.
Although Citymapper won’t work unless you’re in London. The TFL website will, just remember time zones.