Omega won't do it as it's not their internally sourced parts, authorised workshops may do it, but only alongside with a full service, as you pointed out.
Your best option I think is to go with someone independent but reliable - Simon Freese comes to my mind, who is an excellent Omega certified independent watchmaker here in the UK. No idea about the costs, but it's a straightforward routine procedure so I'd be surprised if it costed more than £100-150