It depends on your plane traffic and your antenna. I have a Pi2 which is running at 90% CPU the whole time. It is picking up 150+ planes a day and 1200 mesaages per second. As you are in Birmingham, UK, you have the potential to collect a lot of traffic. Once you get hooked on ADSB, you’ll want to improve your antenna and setup and it may well overpower a Pi2. The cost of a Pi2 vs Pi3 is minimal so I would go for Pi3. There will be enough power to run other applications alongside ADSB too 
Alternative might be an Orange Pi http://discussions.flightaware.com/ads-b-flight-tracking-f21/orange-pi-pc-a-15-alternative-to-rpi-2-so-far-so-good-t36356.html