If you mean the KS3 grades, you can't really equate them to GCSE grades because they are taken at an earlier stage and so of course the overall level is not as high as GCSE. It is possible to use them as indication of how well you will do at GCSE - people get put into different sets based on their KS3 results - but even then, there are so many changes in the two years that GCSE takes, you can't make an accurate prediction.

That said, level 8b is an excellent result and, based on that result you would be expected to be capable of gaining an A* at GCSE. 7a is also very good, and it's very common for people with that result to go on and gain an A* too - anything from B to A* is likely.

You can see a good explanation of how the grades work here.