The price gap
Hotel Indigo and similar premium hotels run £180-£280/night per room in most UK cities. For a 4-person crew, that's £720-£1,120/night vs ~£175 for an entire TNS house. Over a 4-week project, the difference is £15,000-£25,000.
What the premium buys you
A nice lobby. Polished housekeeping. Sometimes a bar that's actually decent. None of these things help your crew finish a project on time or under budget.
When premium hotels are still the right call
Single-night client-facing stays where the brand impression matters. A C-suite arrival the night before a board meeting in central London. Anything where the meeting room and lobby are part of the deliverable. Otherwise: no.