The world of IFR in the US vs. UK has a number of key differences. If you are not already a member it is def worth joining PPLIR for a ton of useful advice and information.
Although someone will tell me the following isn't true, I find it a helpful way of thinking about planning in the UK. Think of three different sets of flight rules, VFR, IFR OCAS, Airways and the following differences
- VFR - as it says on the tin
- IFR OCAS, you plan and fly with a VFR chart, terrain clearance, routing, NAVaid suitability, airspace clearances all your responsibility to check, understand and obtain (in this case you are never cleared to your destination as compared to the takeoff clearance you get in the US). Separation/radar service may or may not be available depending on where you are flying, who is open and their workload.
- Airways - works like IFR everywhere else. You plan and fly with IFR charts, you must have a flight plan that passes CFMU validation and (in my experience) it needs to touch en-route controlled airspace (i.e. an airway). All required clearances are implicit in the process and you will get a separation service (and generally a default RAS when you drop back out of controlled airspace near your destination). However, occasionally you can suddenly be pushed out of the system and moved into type 2 with a cheery 'descend to 4000 feet out of controlled airspace, squawk 7000 free call on route'! At which point the whole job of navigating, terrain separation, airspace clearances drops back to you.