Duplicating the streetAddress property is technically allowed - schema.org properties can be used multiple times - but some parsers may have problems understanding your data.
In the first example at the bottom of http://schema.org/PostalAddress the streetAddress consists of 2 lines of text including both the street number & name and the district of the city. This approach should probably be considered best practice as it's in the official schema.org documentation.
Personally I don't think that an unordered list is the best HTML element to use. A simple paragraph with some spans should do the job:
<h6 itemprop="name">London</h6><br>
<p class="address" itemprop="address" itemscope itemtype="http://schema.org/PostalAddress"><br>
<span itemprop="streetAddress">88 my streetname<br>
City Center</span>
<span itemprop="addressLocality">London</span><br>
<span itemprop="postalCode">W5 4RX</span>
</p>
If you must use an unordered list, then inserting an extra div may solve your problem:
<h6 itemprop="name">London</h6><br>
<ul class="address" itemprop="address" itemscope itemtype="http://schema.org/PostalAddress">
<div itemprop="streetAddress">
<li>88 my streetname</li>
<li>City Center</li>
</div>
<li itemprop="addressLocality">London</li>
<li itemprop="postalCode">W5 4RX</li>
</ul>