You can find more information about entering the UK on these government web pages:
https://www.gov.uk/uk-border-control/before-you-leave-for-the-uk
https://www.gov.uk/check-uk-visa
In general, citizens from the EU/EEA, Switzerland, the USA, Australia, Canada, and Japan will need an electronic travel authorisation (ETA). This list is not necessarily complete and you check for your country of citizenship.
You will be able to apply for the ETA online. It costs £10 per applicant and is generally valid for multiple entries over a 2-year period.
Please note that dual citizens (with British Citizenship) can't use the ETA system (you will normally need your British passport to enter the UK).
If you need an invitation letter or other help, please contact Beth Mountford (ph.ea@imperial.ac.uk) with cc: to Stefan.Soldner-Rembold@imperial.ac.uk.