jon,
U may visit Philippines for 21 days without a visa....then if you enjoy your stay u may extend arrange a 38 day visa waiver extension (P3,000 plus) I don't know how much it is now. I'm from Cebu & my husband just had it process through my Dad's office (travel agency for Php1,500, year2008). You may check the bureau of immigration in Manila.
You may bring Traveller's cheque & have it change to Philippine National Bank of HSBC.
Cash is always convenient. Check always the exchange rate through Tele news, newspapers or nearby exhange rate store on £sterling pounds vs. Php1.
Credit cards is okay but must be expensive & u might have a surprise bill lols!
Declare enough money with you at the Heathrow airport (or any airport) enough for your travel. I forgot. How much we declare when we went for a holiday to visit my family in the Philippines last year. But if you have more to bring, make sure that you have a bank statement with you that shows it's your money. In our case, I brought my husband's business card & the heathrow airport police let us go
It's really your options.
Hope you have fun & enjoy the sun, sea & white sand. Month of August is not the bad to visit. One thing you always enjoy is the sun is always there. Always have water with you. Don't stay too long under the sunshine especially at 12 noon. Try our tropical fruits like mango, banana, pineapple & watermelon. Tastes like heaven. lols. Just like the Cadbury here it's so yummy compared to our Cadbury choc. Even Kitkat tastes heaven here compared the one in the Philippines. I will not talk a lot & leave u all the mystery behind.
Good Luck and have a good stay in the Philippines.