A Transponder is expensive to buy, install and be certified.
Worth considering ADS-B, even a portable one with both an in and out capability. A portable example is the SkyEcho 2 which you can buy for £500 and it doesn't require installation. It's also approved for use within the UK. It can be used with most nav packages such as SkyDemon.
Perhaps someone with a lot more ADS-B knowledge than me will provide much more information than I can, including the various makers and models available.