Back in 1961 a member of Reading Athletic Club decided that he wanted to have a go at fell-running, so he went to the Ben Nevis Race with a few of his clubmates. He won it! His name was Mike Davies, and he later went on to be one of the founders of the Fell Runners' Association. But I wouldn't recommend Ben Nevis as a first fell race!

The Surrey Hills Races in June are quite close to you, but are not suitable for someone with "limited navigation ability" (as evidenced by your describing Reading as "south of London" ). But for real fell races, I would agree with those who have suggested the shorter races in South Wales and Shropshire as good for beginners. The hills are mostly grassy rather than rocky, although they are rather steep in places.

Like Pete S, I would recommend making a weekend of it when travelling to races: enjoy the journey, enjoy exploring an area that is new to you. I have never lived particularly close to the fells (I was in Bracknell for a while), so I often did that.