I'd prefer to implement using spring. But due to given condition no spring. The chosen technologies:
Java 8
Jetty + Jersey
Jackson
Maven
mvn package
mvn install
java -jar revolut-txmgr.jar
Service for create account etc, At-present hardcoded and initialized 18 accounts with account id{1 to 18}
Unit test
Many improvements can be done
If I get an opportunity to build this system for real world, I will prefer to analyze micro services, container(docker) and kubernetes based approach with bare metal hardware in-premise. Alternatively I will also like to analyze deployment in private cloud(openstack).