With that kind of money to invest I would send it to an exchange in a couple of chunks and buy over a period of days. It will cost you ~£8 each time to do a SEPA deposit in EUR but you'll get a better rate than localbitcoins.com. Do check localbitcoins.com first though just to check.
Exchanges that are generally thought to be okay:
http://kraken.comhttp://bitstamp.nethttp://bitcoin.dehttp://bitcoin-central.netI've had SEPA deposits show up in <24h on kraken and 1-3 days on bitstamp.
I suggest doing it in chunks both to reduce risk (exchange can shut down with your money on it like MtGox did) and so as not to skew the market too much by dumping £10k on it all at once.
Every time you buy them, withdraw them to your local wallet; don't treat exchange like a bank, again for MtGox reasons.