If you can still find the Hisense Roku TV at Argos, go for that model. 43B7120 50B7120 55B7120
Otherwise I'd target the new U7Q come Black Friday/Xmas/Jan sales which has features other cheaper TVs lack, local dimming + around 700 nits peak brightness.
The Roku TVs OS is more comprehensive than Hisense's own. Although Hisense's own have all the major apps still. If its OS>Picture quality you should consider TVs from Samsung or LG instead. Samsungs a bit of a gamble with picture quality since its not certain which panel types they use at 43, 49 and 55".
LG uses IPS type panels at 43, 49 and 55" and will fair better in brighter conditions.
All 50" TVs, be those LG, Samsung, Philips, Hisense or TCL use VA panels and will be good in darker conditions.