Don't do it like this. You should rather check for the four main directions, and add their contributions to a velocity vector.
const float speed = ...;
sf::Vector2f v;
if (sf::Keyboard::isKeyPressed(sf::Keyboard::Up))
v.y -= speed;
if (sf::Keyboard::isKeyPressed(sf::Keyboard::Down))
v.y += speed;
if (sf::Keyboard::isKeyPressed(sf::Keyboard::Left))
v.x -= speed;
if (sf::Keyboard::isKeyPressed(sf::Keyboard::Right))
v.x += speed;
Then, you can still address the problem that diagonal movement leads to a higher speed, just divide by the square root of 2 in this case.
if (v.x != 0.f && v.y != 0.f)
v /= std::sqrt(2.f);
The object is eventually moved by an offset which equals to the velocity multiplied with the passed time.
sf::Time dt = ...;
object.move(v * dt.asSeconds());
To compute the angle, you can use
std::atan2(). Please refer to
www.cppreference.com for more details.