I've also heard the Dock Branch plan mentioned as a place to store the preserved 507/8 unit, should it happen. All ifs, buts and maybes at this stage.
Sorry to say but that reply from Angel Trains doesn't quite sit right for some reason: 'the people of Liverpool'? 508111 is likely to be withdrawn relatively early and wouldn't any preservationist want to restore it to an earlier livery? Better to take one of the last units to finish or, if paying attention to numbers, a unit with genuine value over the others such as 507001 or 507033.
If there is no space for a full set, then just save a driving carriage: better than nothing. They will have given over 45 years of service by the time they're withdrawn, so there is as much value (or as little, depending on one's point of view) in saving one as in the case of the 502s and 503s.