Call em up and ask for MATZ penetration and FIS. Then, they will ask for details, so, what, from, going to, height and QNH and reconfirm MATZ penetration and FIS required. You will (may) get a squawk and you may be put under a radar service, so you are under their control. Depending on height, you might be asked to change to avoid conflict with incoming/outgoing. I've found both RAF and USAF always very helpful and plane spotting from 2000' is fun!! Beware the USAF bases, they may come straight back with a squawk, then identify you and then ask for details.
That's my experience and rarely refused MATZ penetration, but be prepared to reroute if they think there is conflicting traffic, although they may say 'resume own nav', there's no harm in asking for a QDM to your next waypoint if unsure.