To add/clarify to previous comments
You can read tons of back threads on this. But quick summary of reasons to be N-reg
1 - The aircraft may have modifications/equipment that are not approved in the UK or the cost of change may be excessive.
2 - Modifications to be on the G-reg may be expensive to remove hence reducing its resale area to Europe (excluding the US market may reduce resale value/speed)
3 - If you don't have a JAR IR, it is less time consuming and requires no mandatory ground study programme to get an FAA-IR (slight simplification - FAA/IR is good in N but not G)
4 - On N aircraft Service Bulletins are not mandatory but they are on the G-Reg. Many of the SBs are sensible but some are unnecessary and some actually increase the risk of problems.
Finally, there is simply the cost of changing. This needs to be set against the ongoing cost of the Trust (unless you are a US citizen).