These radios are inappropriate for your needs. The GP340 is a business radio that can only legally transmit on the frequencies assigned to the company who licenced them, and - even more important - within the geographical area assigned to that company. By using radios on unlicenced frequencies, you are risking not just interfering with the original company’s business operations, you could be interfering with public safety channels too. This can cause serious problems and may even become a life-threatening situation. For that reason, the regulating authority in the UK takes this stuff very seriously. With satellite tracking, they can find offenders quite rapidly.
Either return those radios to the person who sold them to you and pick up some good PMR radios, or apply for your own business frequency and have a radio dealer reprogram them to your assigned channels. Quite frankly, whoever sold you those radios should have disclosed the fact that they are useless unless you get them reprogrammed to your own licenced frequency.