Taxcalc is a good application, we use it for both our CT600 and SA100 Self Assessments, so I would have no hesitation to recommend them though we have never needed to contact their support team so cannot confirm how good they are. The CT600 does not really require and foreign currency postings as far as I am aware, any values reported would have to be translated to £GBP on the return.
We were in a similar position where we could no longer use the free online option for submitting both accounts and tax for a micro-entity because we started investing some of our company cash into financial assets (stock market) and were forced off that platform and found Taxcalc.com to be good for our needs as it allows us to file our CT600 as well as SA100 returns.
We have not really changed how we report the balance sheet and have included the financial assets as part of our fixed assets total.
If you are filing your accounts using FRS 105 (Micro-entities) then you do not need to submit a P&L, you only need to file a Balance Sheet (currently) and a very limited set of notes. You generally do not have to disclose the specific amount of FX gains or losses in the filing.
However saying that, I would think you still need to use correct HMRC published rates or actual bank rates depending on the transactions and correctly account for gains and losses as they would impact your profits and therefore your taxes.
Under FRS 102 (Small, Medium to Large Entities) you have more strict disclosure requirements and must include notes on the accounting policy for translating foreign currency items and how you treat the resulting gains or losses. You must disclose the total amount of exchange differences recognised in the P&L for the period (unless you are a "Small" entity using Section 1A, where this disclosure is encouraged but often technically optional unless it's necessary for a "true and fair view").
How currency gains and losses or revaluation of contracts in foreign currency are worked out or translated would vary depending on whether you are using FRS 102 or FRS 105, so you have to read up on this if you deal in foreign currency accounts, especially if you don't use an accountant.
For now you should be able to file your accounts directly with Companies House separately and then use a separate application for your CT600, however going forward Companies House will also require you to use software to submit the accounts, so you will need to find products that do the accounts production as well as the tax returns, or use an accountant who can do all 3 things (Accounts Product, Corporation Tax and Self Assessments)