If they have a dry place to retreat to I believe some of the English keepers allow their Russians to live outdoors and even hibernate outdoors over the winter. That said, different parts of England have different micro climates.
I live in the Pacific NW. Our latitude is a bit further South than yours (45d, yours is 51d).
Our weather is similar to many areas of the UK.
If you create an outdoor enclosure with a cold frame or small greenhouse, especially if it has an electric source and thus a basking light, your tortoise can stay out year round April to September/October.
You can then overwinter the tortoise inside, or decide to brumate him (a whole different topic).
As far as temps, Russians (Horsfields) can tolerate colder night temps into the upper 40's, IF they can bask to warm up. Either artificially, or on a sunny day when the sun comes up and warms up gravel/paver areas.
You can't just judge keeping your tortoise outside by air temps, as the ground temp on gray gravel or pavers can be a lot warmer. I have had Russian tortoises basking when the air temps are 10c (50f), but their shell temps were 29c (85f)!
So....if you don't have a cold frame/GH, check the ground temps in the sun or a bright cloudy day. Their shells are very good at absorbing warmth.