Can't find the online document but the last 319 is planned for 2017. Only units running around on Thameslink in 2018 are 387s and 700s.
Govia has publicly stated Thameslink fleet replaced to be completed by the end of 2017.
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June 2018 is certainly the last delivery of the 700s. The full fleet is needed for the May 2018 timetable, so something will be staying behind. You're more likely to be right than me!