This isn’t unique to Manchester so whomever is on the “it’s discrimination” line should give that up straight off. The UK DFT has a certain set of “More Stringent Measures” which mandate re-screening of any passenger who has arrived in the UK from overseas before boarding another flight either domestically or internationally. It doesn’t matter where they have arrived from - this rule applies.
It is a major brake on the UK being used for any transit flights. A handful of operators choose to do it, but there can be no doubt that this limitation deters countless others at a variety of UK airports. Note that Air India, Kuwait and others all stopped using Heathrow as a transit point some years ago after this came into force.