F5 VIP forwards to all pool members whilst retaining source IP

If you are only using UDP you could try to set Address Translation to none. Alternatively you will have to add an irule replaces the source address, but that'll be a task and a half, as UDP does not seem to include local address.

The appropriate way to do this is to make F5 the router for the servers it loadbalances for. This will also make it possible to retain actual IP adress of TCP connections (as long as the server and client aren't on the same network).