Using a plate does not upgrade the vest to a higher level. There are Level III with/without plates or Level IV with or without plates. Adding the plates does not raise the level.
Israeli vests are good. I work a lot with Marom Dolphin Sedo Domain Parking - Sedo GmbH and understand they have a reseller in the UK. They are also suppliers to the police and IDF SF.
Hope that helps.
No probs on response ,thanks for clarification on Levels ,I think I need to research some more!
Sorry about the delay in return an answer to your questions. In a nut shell, there are multiple Body Armour Standards, NIJ, HOSDB (UK), European, Russian etc... An example of difference Standards is the Back Face Signature (BFS) the depth of deformation of the clay backing to the armour being shot, NIJ is 44mm and the HOSDB is 25mm.
Each is slightly different from the other. It all boils down to what is the primary threat; Pistol, Assault Rifle or Sniper, once you have established that, then look at the different terminal ballistic standards. Select one that provides the best protection for your threat. Then look and ask around the community what other users think of the Body Armour they are wearing. Once you have selected 3 or less models, approach the manufacturer for the ballistic testing data for that model of body armour, and any independent testing conducted on the armour with special threat ammo.
This may all seem alot to do, but in the end how much is your life worth. If you want the latest NIJ or HOSDB standards PM me and I'll email them to you 3 files in total.