No, you don't have to purchase anything once you're on board - and I wouldn't, if I were you! The lottery tickets are just an extra way of making money - purchase is optional (you just get annoying announcements about them). Likewise, the food costs far too much and isn't that great - but you don't have to buy it and they don't stop you taking your own food on to eat if you really have to eat during a 2 hour flight :)
I imagine they make £1 per passenger on the actual ticket, if you see what I mean. They make their profits on the high charges on everything else.
Btw, although I haven't flown that route, I've flown Ryanair from Stansted to god knows how many destinations (mostly Italian) in the last 5 years or so, and it's been absolutely fine. You just have to be prepared for the following:
- Charges for anything they can think of: they charge you to check in a bag, and they charge you more if that bag is over 15kg; they also charge an 'airport check in fee' - which you can only avoid if you travel with hand luggage only and check in online. (Hand luggage limits are a bit smaller than on some airlines and there's a weight limit of 10kg - and they really enforce the ONE hand luggage bag only, handbag has to fit inside that). Oh yes, and when you book, they automatically add an insurance fee, which you can take off, and there's a fee for whatever means of payment you choose. Happy days.
- No assigned seats, so people say "it's a scrum" when you get on the plane. Personally I've always found that extremely exaggerated, it's not that bad, but then I don't really care where I sit so I don't get involved (if you really have to sit in a certain place, you can pay - oh yes, more fees! - to get 'priority boarding', which means in theory you get on the plane before this supposed "scrum")
- Minimal legroom and really awful decor ;) Cabin crew who aren't exactly charming and may or may not speak English. No 'mod cons' on board (including no magazine pocket in the seat in front, which is more irritating than you might imagine ;)), annoying ads for lottery tickets, food, blah blah blah.
- Little to no customer service if anything should go wrong. Very little chance of a refund, ever.
Basically read their Ts and Cs very closely, treat it like a bus, be careful with your luggage allowance, and take an ipod to zone out of the announcements. And don't rejoice in the cheapness of the fare until you've got to the very last stage of booking, there's always something else they can find to add on ;) But seriously, even with all their cheeky extra fees added, they're still usually cheaper (for when and where I'm flying) than any other airline, so I can put up with the nonsense, personally.