Tandem are offering 5% easy access according to MSE. Not read into it yet, but the only downsides on first glance is that you have to manually tap a button to increase the rate from 4.65 to 5.0 (wtf?!), and that the 0.35% boost only applies for a year.
Very tempting, but I've never heard of them.
Shawbrook are offering 4.63% but it sounds like you have to get in touch to request moving to a new product every time they offer higher rates, which is a bit of a pain.
Chip get a good rep on here, but are lagging at 4.51%.
Marcus have the top up interest rate too where you have to tap a button but you've always been able to renew it so I assume Tandem will allow this too.