Harrier share the same chassis as the Lexus RX series. But features and specification on Harrier and Lexus us different. Harrier comes in 2.2L and 2.4 L form while the Lexus RX is with a V6 engine 3L or 3.5L. RX is equip with more luxury and high end stuff example Mark Levinson sound system and electric seats...etc Both the Harrier and RX comes in 2WD form or AWD.
It does not matter if it's UK or Japan Spec. Lexus RX and the Harrier was sold by different distributor in Japan and the RX was mainly for export market. When I bought my Lexus CT I got ask the technician and saw Lexus got some information why grey importers specification is different from those model sold by Lexus authorized showroom. Like the thickness of the radiator is different.
I thought engine options are the same with Harrier and RX. I've checked that only the first generation have the 2.2 and 2.4 straight 4 with one 3.0 V6 option. The second generation is only available in V6 which is 3.0, 3.3 and 3.5. The third generation is no longer called as Harrier already but the recent fourth generation is called as Harrier again. Probably because during the third generation, Lexus has already entered into the Japanese market. Before that, all Lexus models are sold as Toyota in Japan if I am not mistaken. Celsior, Aristo, Altezza, Windom, Harrier, Soarer... Not offence here, pardon me if I sound a bit rude.



