Summer Internships can only start after the summer teaching term has ended. In 2017 this ends on 30 June so the earliest date an internship can be considered for summer 2017 is 3 July.
Earlier dates are not possible because of staff UG teaching commitments.
The Astrophysics group does not have any internal funds to pay for summer internships. You must therefore seek funding yourself. If you are an Imperial student this might be via the UROP scheme. Other fuding possibilities include the RAS internship schemes.