Pink Sheets is not a stock exchange per se, and securities traded through it are not as "safe" as the ones on a stock exchange regulated by SEC. Many companies are traded there because they failed to comply with the SEC regulations, or are bankrupt or don't want the level of reporting to the public that the SEC regulations require.
Since you're talking about an ADR of a company traded on LSE, it might be much safer that other, "regular", securities, but still it means that you're buying an unregulated security (even if it is of a company regulated elsewhere). Notice the volume of trades: mere thousands of dollars per day (in a good day, in some days there are no trades at all). It makes it harder to sell the security when needed.
Why not buying at LSE?