Potential registrants need not register before they know whether they have received a FEMS Congress Attendance Grant or their abstract is accepted. They will then still be in time for their visa application. According to the UK government website, you can apply for visa earliest 3 months before you need it. 
Invitation letters can be requested through the online registration.

Please note that official letters of invitation can only be given to individuals who have registered and paid the registration fee for the congress. All requests will be considered individually.

If a participant’s application for visa is refused, the Congress Secretariat will refund the paid registration fees, provided the congress secretariat has received a copy of the refusal letter before 15 June 2019. We advise you to apply for a visa as soon as possible (after 7 April).