1. Because LH values their top tier fliers way more than UA does?
2. Because UA doesn't have the space to offer such a service at their Polaris Lounges and/or United Clubs to make it consistent across the network?
3. Because UA is cheap and won't spend the money for this (signage, staff, etc.)?
4. Because UA doesn't have Global First any longer?

It's some combination of the above or probably all of them.

UA still sort of has a separate GS area in the LHR lounge (and maybe some others) - but 1Ks are often invited to use the same area at LHR when the lounge is busy (which is most days).

-RM