Putting borrowed money into an SIPP
You're creating more liabilities for yourself in the future, although yes this could definitely be a profitable move for you. However, some small mistakes you made, from what I can see using the tools at Hargreaves Lansdown. The first, is that the government relief would only be 20%, not 60%. The second is that the tax relief goes directly into the SIPP, it's not something you get given back to you in cash. In order for this to be worthwhile, you need to be sure that you can make a post-tax gain of more than 3.4% on this money per year - which should be very feasible. It sounds like you have enough security that you could afford to take this risk.