Pros
You might be there long enough to get some redundancy money - only statutory minimum though. You could be working with some decent up to date tech but it all depends on which client you'll be working with. See the cons. There are some great people there, but the management is where the problem lies. Pay is competitive but not great.
Cons
Your experience depends on which client or service you could be working with as Encompass London is a mishmash of different companies and clients squished into a West London office, buying other companies is probably the only way they remain profitable. You could be really unfortunate and work with a toxic core of staff where bullying, harassment and childish vicious behavior seems the norm. This is condoned by the company and all problems are simply explained away as 'this is what Encompass is like'. This has lead to a mass exodus of technical staff while the old boys club can just do whatever they like. Running down the headcount while expanding the client base seems the plan, probably to sell the company at some point. Don't rely on these guys for any security or make long term plans. Unrealistic expectations regarding workload, this is a very top heavy organisation with many managers and few workers. No leadership from the top. Over reliance on freelancers and poor project handover to technical staff.