It's not the TV that is a problem. It is the input signal strengths.
You either have an older antenna (or a wrongly aligned one) that is picking up the Welsh channels from their transmitter, or receiving signals off the back of the reflector on the antenna.
This is a technical issue that may right itself after changeover is completed. You will possibly sort it by fitting a high quality, more directional antenna, a special group amplifier or a filter.
Either way, you will need advice from someone local like installer who can look, check and see what's happening.