I am assuming you have not been requested to provide proof of accommodation (on the basis that if you had been you would have said so!) so refer to the GOV.UK guidelines where in Section 4, which is:
documents you should not send unless specifically requested
there is, amonsgt other, items:
The rationale will be that once allowed entry it matters little where you stay, coupled with it is very easy to cancel a booking at no cost as soon as it has been presented for a visa application.
However, if you have been asked for proof then I can see no reason for a hostel via booking.com to be any less acceptable than a hotel booking.
Anyway, you seem to have an excellent reason ("premise") to be visiting the UK and provided you pay attention to the forms and their requirements your chances of obtaining a visa seem very good, regardless of accommodation.