You can, but you cant use First Capital Connect trains to London Bridge (for South Bermondsey). Would have to take a Victoria train, changing at Croydon. Would probably be cheaper getting a FCC only Travelcard from Brighton - as you can then use Southern in the London area.
Last ones from Watford Junction at 23.29 - would easily make that.
Link to buying the Daysave here - DaySave Information - Southern railway - £24 for 4 of you using Southern ONLY.