You cannot get local currency outside of Nepal. Therefore you need to take cash to exchange in Nepal or use one of the many local ATMs.
Please take at least £20 per day cash plus £35 for an entry visa to Nepal (or the same value in USD's, Euro's, or any major currency) as ATM cards may not work as your bank can block them for security reasons. In Kathmandu there are many ATM's provided by Standard Charter and the local banks and there are also a few ATMs in Pokhara.
If your bank blocks your card you may not have the time to unblock you card before leaving Kathmandu for your trek. It is advisable to tell your bank that you are going to Nepal so that they are less likely to block your card. Additionally if you use your bank card at the airport your bank knows you are leaving the country and this reduces the risk of your bank blocking your card.
In Kathmandu most people will have the option of visiting the ATM or visiting the money changers. Please choose which option you prefer.
If you arrive in Kathmandu after 6pm and are leaving for your trek the following day you may not have time to visit the money changers. This reduces your options to using one of the many ATMs or exchanging cash at your hotel (worse rates than the money changers). If you are travelling to Pokhara you will have a second opportunity to use an ATM or visit the money changers.
Trek with the famous Sherpas and follow in the footsteps of Sir Edmund Hillary to Everest Base camp.
Get some inspiration and choose an epic adventure suitable to you!
Watch a short clip to see the wonders of the Himalayas to get some inspiration.