Splitting across several platforms is what I do. Partly to spread risk and partly just out of curiosity of the various platforms. I quite like Assetz Capital myself. There was an attempt at a discussion on favourite P2P platforms here but it never really got going.
As to your question about avoiding fees. For some reason I can’t load the Zopa website at the moment to check, but I don’t think you can specify a fixed period for investing. You would probably need to turn off auto-invest in your account settings and as old loans mature, new loans won’t be purchased. However, it would take a long time to get out of your investment this way. Some of the loans are several years in duration.