In most cases, yes — a plug-in or inline RCD adaptor can add protection to specific circuits, but the proper solution is a full consumer unit upgrade.
🏠Homeowner view
If you have an old rewirable fuse board (ceramic fuses) or an early MCB board without RCD protection, you have a few options. For individual circuit protection, a socket RCD adaptor (plugs into your socket and adds 30mA protection to anything plugged into it) or a plug-in RCD can protect specific appliances cheaply — they cost around £10–20. For outdoor circuits, you can add an in-line RCD socket. However, these are partial measures. The proper and safest solution is to have an electrician upgrade your whole consumer unit to a modern one with full RCD or RCBO protection — this typically costs £600–1,200 and is well worth the improved safety. If you have an old consumer unit without any RCD protection, any electrical work done on the property requires upgrading to current standards.