The cheapest through ticket from Bristol Temple Meads - Folkestone is an advance ticket at £35 each (£210 total), departing Bristol Temple Meads at 8:35am, and arriving Folkestone West at 12:28 or Folkstone Central at 12:30.
It is the same price leaving Bristol Parkway at 8:12 and changing onto the above train (but you need to decide before booking, as your ticket will be valid from one or the other but not both)
If some or all of the travelers will be making more rail journeys together in the next 12 months, then it may be a good idea to buy a 2 together railcard. This will reduce the ticket price from £70 per 2 people down to £46.20 per 2 people - but this saving will be outweighed by the purchase cost of a railcard (£30 in station or £27 online) unless either further journeys are being made, or you have tesco points which you want to use up (they're currently £15 worth of points, and savings may be available in the "flash sale" for clubcard/railcard purchases which is advertised as beginning on 3rd March, but without details)