There are a number of things you should consider when searching for an Umbrella Company provider.
All UK based PAYE Umbrella Company providers are governed by HMRC, which means that no compliant Umbrella Company can provide you with a higher amount of take home pay than any other. It is better to look for a provider with a flat fee as these ones are potentially cheaper.
Customer support and ease of access to submit your timesheets and expenses are also factors to consider.
You should always ask for a full breakdown of all deductions, and if there are any hidden costs or penalties, such as set up and leaving fees.