This question was asked in last year's AMA with THL UK. It was also addressed in THL UK's marginal funding post this year. In summary:

• THL UK receives an annual grant from THL, but they are trying to reduce their dependence on them. In addition, because of US lobbying laws, THL's grant to them can't be used for legislative work (for instance, the recent Frankenchickens case) or lobbying work (for example, lobbying for fish welfare in the UK Parliament), and one might believe that such work has a greater chance of succeeding in the UK than the US.
• THL UK likely focuses more on fish welfare than THL as a whole.
• THL UK also engages in activities in other parts of Europe, not just the UK.
• UK donors may be more likely to donate to THL UK as they may be able to claim Gift Aid, although they could donate to THL via EA Funds and might be able to claim Gift Aid there.