Maybe just pick a town/city and then get in touch with a hotel and try and do a group booking. If you just want to go to a club then you're probably best doing it yourself rather than through a company, like you said they just want to get your booking secured and badger you until you book.

Why not book everyone into a Premier Inn or Travel Lodge in London and just have a night out in the city? 

Sounds like you just want dinner and a night out - why not do that?

IDEA IDEA (Love hen do planning)

for my friends hen do we went to Brighton (by train) early so we got the whole day all ages were invited brides mum mother in law to be friends aunties

we went for dinner at JB's Dinner along the front

watched the dream boys

then older hen do guest went off and did their own thing and the rest of us stayed out clubbing. 

the next morning the people that wanted to stay down there got up and went shopping and lunch (some hen do guests couldn’t afford to stay  the night so got the last train home) 

but my friend did want fancy dress and her boyfriend is football mad so we all went out in skirts  or shorts long socks high heels(obv course) and Football shirts and whistles) to pay tribute to her H2B

The hotel was about £20 a person  (seafront travel lodge)

dream boys was about £40 a person

i have done the dream boys in london for another hen do but didnt stay so dont know how much it would cost!

i really hope this helps

Travel lodge and the likes are definitely a good idea. Using a company for one night is possibly not going to be cost effective. Instead you could book something independently. There is so much to do in London. You'll be spoilt for choice!