Best bet is not to exchange cash.  Just withdraw what you need every few days from the ATM — you’ll get the best exchange rate and usually pay only a minor fee.  The airport and your hotel are usually the worst places to exchange money.  They usually give the worst exchange rates and/or have the highest fees and commissions. 

If you have to exchange cash the best place to exchange is usually a bank, but it’s often inconvenient.  There are sometimes lines and banks open late, close early and are often closed over the lunch hour. 

i think you should get most of the cash you are going to spend at home

i go to my bank, withdraw the foreign currency, in your case British Pounds, and keep that cash right beside your passport – in your money belt right near your groin.

It will rain in London in April, that’s almost certain – but on the other hand it may not. It may be quite warm, it may be very cold, the one thing that you can be certain of is that the weather will be unpredictable, and probably it will also be changeable.
 
Pack clothes that will layer, pack a waterproof, two pairs of shoes.

  This will depend on where you live.  In the US, it’s usually incredibly expensive to get foreign currency unless you live in one of the major international gateways.  Local banks in my area usually charge a 3% commission on top of a fairly poor exchange rate.  If they don’t have any of the currency you want on hand there’s an extra fee to ship it in.  I prefer to hit the ATM at the airport where I land — even if I get hit with a double ATM fee.  I always carry a couple of hundred dollas in US twenties.  If there’s a problem finding or using the airport ATM, I change those at an airport exchange booth. 

Thanks for all of the advice. I was thinking of bringing travelers checks as apposed to hitting up ATM. Is this wise? I’d rather only use ATMs if its an emergency situation.

Traveller’s checks are pretty much obsolete. You may find it hard to cash them.