You cannot insist. However, have you explored whether there may be a way to compromise? For example, all employees with 26 weeks' service are allowed to ask to work flexibly. You could make a flexible working request to cut your hours so that you don't work on Fridays or so that you work only part of the day or swap shifts with a colleague.
Your employer is not obliged to agree, but can only refuse your request for good business reasons – for example, because there is nobody available to cover your Friday shift.
Another possible solution would be to ask for arrangements to be made for a break and somewhere to pray at work. Other colleagues could also benefit from this sort of arrangement, which would be less disruptive than your leaving site.
Obviously, your request in this case is linked to your religious beliefs. Your employer will want to be careful not to discriminate unlawfully against you. However, it is not unlawful discrimination to have a policy in place – such as requiring security guards to remain on the premises during working hours – even if it impacts negatively on workers of a particular faith (such as someone who wants to attend Friday prayers), as long as the policy has a legitimate aim and is executed in a reasonable way.
The law says your employer must act 'proportionately' (i.e. reasonably) if they want to avoid accusations of discrimination. In this case, it would be proportionate to listen carefully to your proposals and to see whether a compromise can be found. A good compromise, depending on the resources available to your employer, might be an onsite prayer room.