No. Random Number Generation is the process by which a machine selects a specific number from an available set. This number is then used to determine future events. Many slot machines rely on an RNG to pick the appropriate symbol for that particular slot. The UK Gambling Commission oversees and tests this process to ensure fairness. It looks like casinos are unable to change the RTP for an online slot. Games will have their theoretical RTP already set by their developers and these values cannot be changed by the casino. However, if an operator has passed all necessary inspections and licensed this specific game, you would be able to see what that specific slot's RTP is. Progressive jackpot slots would generally have lower RTP levels than the typical random number generator games.