Aviva low cost endowment

Does anyone have any idea what kind of terminal bonus I can expect

No-one on the internet can answer that. There are variables unknown to us. i.e. what is your target growth rate (a plan with 4.4% target growth rate will have a different amount to one with 7.7%). What version of the endowment is it? (plans were not all the same, different versions came out with different charges). Who was the original provider (Aviva is a catchall for many different insurance companies. Each offering its own versions of endowments and with different WP funds at the start).

Most LCE plans dont suddenly get a chunk added to them on maturity. They accrue the final bonus as they go along. So, your plan will already have a final bonus on it (which can change daily). Your IFA will be able to see this and tell you what it is and if you get the right person at Aviva on the phone, they can tell you. A crude ballpark is to look at the difference between the current position and the surrender value (although that can be distorted by surrender penalties if any).

Also, do not forget the MEP. That needs to be added on as well, if you have one (and many Aviva endowments do).