No, it's quite clear that the 800 staff are based at Hoddesdon:
It is also envisaged that the new site will accommodate approximately 800 additional staff for the UK domestic market and other European clinical functions currently based in MSD in the UK’s Hoddesdon headquarters
Quite coincidentally of course, the Hoddesdon R&D site has been sold to Chinese company Pharmaron, as announced a year ago:
www.pharmaron.co.uk/ckfinder/userfiles/files/Pharmaron_Press%20Release_26_09_16.pdf
Under the deal, MSD will remain on site on and lease-back the main office buildings.
MSD believes that locating a research facility in London will expand MSD’s opportunity to engage with leading researchers in the UK and Europe.
www.outsourcing-pharma.com/Article/2016/09/27/CRO-Pharmaron-and-MSD-reach-shared-site-agreement
MSD had already closed its Hoddesdon R&D facility in June 2016 following a global strategy review. MSD has also closed plants in Ireland, so I doubt the EMA moving to Dublin would have made any difference to its strategy.
Pharmaron completed the Hoddesdon R&D facility deal in February this year.
So not quite such a straightforward story as it first appears, as MSD were already planning to move. Just convenient timing for the Government.