Nothing stopping them leasing the voyagers before Grand Union do...
Actually you will find there are some other classes where current leases expire before mid 2026 and currently is no public record of being extended by current user
In theory if they went to a LeaseCo and outbid an existing user of an IET or similar, who hasn't yet renewed, could they grab few of those.
Presumably any operator who hasn't committed to leasing their fleet through to 2030 could find someone else has leased it from end of current lease, unless they have guaranteed renewal rights (which I don't think exist anymore unlike the early leased stock).
When we had the speculative thread about stock still around in 2044 few weeks back I predicted that many of the pendolinos would be snapped up by open access operators when HS2 stock arrives, I might have just had right idea, but wrong details. Unless they have been extended all the class 390 leases expire March 2026.