If all you're looking for is details of how to calculate your maximum permitted tax free lump sum, you should not need to incur the expense of going to an independent financial advisor.
From the information you give, the lump sum you will be able to fund through AVCs is as follows:
(a) 1.5 times your final remuneration
less
(b) the tax free lump sum you will receive from your current scheme
less
(c) the tax free lump sum you received from the UK scheme.
Final remuneration can include all earnings from your employer including BIK, but fluctuating earnings will need to be averaged over a period of 3 years. You can choose a longer period than 3 years if this will produce a better result.
Your employer's HR/pensions department should be able to tell you how much (b) is likely to be.
If you are in the public sector, you may not have a choice regarding AVC provider. However, if you invest your AVCs in a cash fund, you should be able to target the amount required to a reasonable degree of accuracy, but watch out for the charges applying as these can be quite high.
Hope this helps.
Homer