The RX550 is a brand new card (running on Polaris). However, the 560, 570 and 580 use the same silicon (14nm Polaris) as the previous generation (460,470,480) with higher clocks (Typical AMD - 
). The 560 is an exception; AMD decided to unlock 4 extra compute cores (Which a developer and overclocker did for the 460 last year) so a performance boost would definitely be noticable even with lower clocks.
I wouldn’t reccomend the RX550 for £80 because for £110, you can get the 4gb 560 which definitely be a performance boost (Probably around 2x performance) and only for a £40% price increase it’s probably worth the extra cash.
To be honest, even the RX 560 isn’t worth the cash! Don’t make the mistake I did buying a 460 to test. http://zcashbenchmarks.byethost16.com