How many bedrooms? I always think you risk devaluing the house if you remove a bedroom and swap it for a bathroom. If it's a 4 bedroom cottage, then converting a small room to a bathroom probably wouldn't make much of a difference in terms of price. But if it's a 3 bed or a 2 bed, dropping down to a 2 bed or 1 bed respectively would make quite a bit of a difference, but also seem a bit strange having 2 bedrooms with 2 bathrooms, or 1 bedroom with 2 bathrooms.
The fact that you're concerned about dedicated parking means that it is a requirement for you. Parking down the road might be okay short term, but i agree about the concerns of having to rent a parking space down the road, the owner could sell up at any time. Also at £600/year, if you're planning on staying there long-term, after 5 years you'd have effectively paid for putting a driveway in and lowering a curb.