What Boat for cruising?

Hi.

We're looking for a boat that will sleep at least two and have usual
self contained facilities for cooking etc, and;

Must do lots to the gallon (Diesel?)

Usable around the coastline of Britain and the odd dash to Europe,
plus inland rivers and be suitable for living on long term, say weeks
or months at a time.

Can one just park up anywhere whilst cruising (for the odd night/day)

Fancy doing some touring really but instead of a car and caravan we'd
like to do it in a boat.

Least amount of hassle and as reliable as possible, not to mention
reasonable price is what we're after.

Possible or a bridge too far?

All input much appreciated.

Thanks.