Registration & Visa application

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.

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. According to the above mentioned website, you should get a decision on your visa within 3 weeks, so this should be feasible unless you register 24 May or later.