`MerchandiseHighSectionSwitch` on `/uk` front

[ioannakok]

Support MerchandiseHighSectionSwitch that moves the merchandising high section lower on a UK front, to coincide with an ad campaign. See frontend PR that implemented it: https://github.com/guardian/frontend/pull/23677

Once this is implemented we need to make sure we're taking the switch into account in the logic that decides the merchandising high position. See frontend logic.

Before (switch off):

After (switch on):

[alinaboghiu]

Ashish says we don't need to do this

[ashishpuliyel]

We don't need this functionality. We should just take the switch out.

[rhiannareechaye]

This is a small ticket