That is a lot of jitter, and anything above about 25-30ms will make VOIP and video calls really not great.
Can you run the EE SIM in your phone and do the same speed/jitter test outside, and then again inside still using the phone?
The RSRP you had is basically useless, so an antenna should help a lot, it was probably bouncing all over the place.
Will try that this weekend. Yeah the RSRP is awful, surprised I even get 150Mbps and may well be the cause of my latency/jitter.