The cheap Club fares to HEL provide some great opportunities for those not averse to back to backs and willing to originate from JER eg
Ticket 1 18 March BA2773 JER-LGW
19 March BA794 LHR-HEL
25 March BA799 HEL-LHR
25 March BA2776 LGW-JER
Cost on ba.com using multicity pricing 287.91 GBP
Ticket 2 19 March BA799 HEL-LHR
19 March BA2776 LGW-JER
24 March BA2777 JER - LGW
25 March BA794 LHR-HEL
Cost on ba.com using multicity pricing 338.92 EUR (286 GBP)
So 480 TP for 574 GBP at around 1.20 GBP/tier point
Whilst switching airports is painful assuming you are London based it should be possible to break up with overnights in London each way.
Note that if you try and price a regular JER-HEL roundtrip on ba.com these fares won't show. Instead look on Google flights and then build up the fares using the multicity pricing functionality