Firstly, the currency has not been "kept" overvalued, because the British Pound floats freely. It has been since 1992 and as far as I'm aware the BoE hasn't made any interventions since then to either push up or push down the Pound. (maybe during a crisis, like 2009, but certainly not as a regular policy)
Second, the structure of country's economy (in which you're probably massively overestimating the contribution of money laundering in the UK, but that's another discussion) has got nothing to do with whether its currency is overvalued. Or undervalued, for that matter. A country's economy activities could consist entirely of cow dung and it'd still be possible to have an overvalued currency. I also don't see what benefit having a overvalued currency would have for money launderers.