With Hitachi having made good progress on the outstanding 800s 801s and 802s there shouldn't be much of a lead time at all. With the future surplus 185 seemingly not going to a new home in a hurry the ability to retain them as an alternative to MK5s adds up capacity wise (185s not going to RoI also makes more sense if this is the case.That's still going to be a fairly substantial lead time. Do they really think the Mk5As will take longer to introduce than having more 802s delivered, for which bodyshells have yet to be constructed? And if so, why are 2-3 units still being deployed? That doesn't make a great deal of sense to me, even if they do ultimately benefit from a slightly more standardised fleet.
First also don't need their East Coast open access 802 for another 18+ months hence the ability to swap those to TPE spec for quicker delivery makes sense.
First also have outstanding options for 802s they haven't used yet.