While there is no direct way that I have found to donate to the Fund via credit card as of today, there is direct contact information for their office available on the website. If I am able to find a simplified way for brethren outside of the U.K. to donate to it, I will update this posting as soon as possible.
Lacking a direct link to simplify donations, I have used TransferWise, which is a very low cost service to transfer money directly to foreign accounts via your credit card or bank account. It also seems to be the best way in regards to the exchange rate. I'm not shilling for them, just saying it's successful, simple, and a global service (Richard Branson was one of its founders, partially out of frustration over international money transfer costs).
Be sure you specify the East Lancashire Masonic Charity as the recipient, use the email, address, and bank account information from the Province's web page, and put "Manchester Bomb" in the memo line (looks a bit odd, but it won't accept anything longer).