they saw no need to get a British passport.
They don't really need to nowadays - Polish citizens can live and work in Britain and vice-versa. The only advantage of a UK passport for Poles is if they want to visit the US wthout a visa.
how did you leave communist Poland in 67?
I didn't - I'm originally from Britain but live now in Poland. There are tens of thousands of Brits there and that number is increasing faster than anyone can keep statistics. I was in M&S in Warsaw not long ago and realised that the only Poles there were the staff.
Though there are plenty of Poles who left around that time - some to join family, some were given visas to leave, some took holidays or went on business trips etc and didn't come back and a whole raft of people left in 68 due to political conflict. These were generally highly educated and skilled people who were snapped up by whatever country they settled in.